Vedas not being eternal is not proved from the above point.
For example: RigVeda 10.129 states Gods coming after the Creation. Now, Veda Samhita contains most of the mantras of Gods. Going by the same logic given above, it also shows Vedas are not eternal.
However above is not the case. It doesn't prove Vedas being non-eternal, because Vedas propound cyclic nature of Universe.
सूरय्याचन्द्रमसौ धाता यथा पूर्वमकल्पयत् I
दिवं च पृथ्वीं च अंतरिक्षमथो स्वः II RigVeda 10.190.3 II
"The Ordainer created the sun and moon like those of previous cycles. He formed in order Heaven and Earth, the regions of the air, and light."
So, such things mentioned in Vedas are also eternal due to Cyclic nature of creation.
For example: if Vedas state Vishwamitra heard Gayatri Mantra. Now, objector objects that Vishwamitra has birth and before Vishwamitra there was not that mantra hence Vedas are not eternal. This is wrong because the names mentioned in Vedas are like titles in this case. ie. Gayatri Mantra is heard by person named Vishwamitra in every creation as discussed in answer here. As quoted in the same answer from Sri Bhasya by Sri Ramanujacharya:
As words such as Indra and Vasishtha, which denote gods and Rishis, denote (not individuals only, but) classes, and as the creation of those beings is preceded by their being suggested to the creative mind through those words; for this reason the eternity of the Veda admits of being reconciled with what scripture says about the mantras and kândas (sections) of the sacred text having 'makers' and about Rishis seeing the hymns; cp. such passages as 'He chooses the makers of mantras'; 'Reverence to the Rishis who are the makers of mantras'; 'That is Agni; this is a hymn of Visvâmitra.' For by means of these very texts Pragâpati presents to his own mind the characteristics and powers of the different Rishis who make the different sections, hymns, and mantras, thereupon creates them endowed with those characteristics and powers, and appoints them to remember the very same sections, hymns, &c. The Rishis being thus gifted by Pragâpati with the requisite powers, undergo suitable preparatory austerities and finally see the mantras, and so on, proclaimed by the Vasishthas and other Rishis of former ages of the world, perfect in all their sounds and accents, without having learned them from the recitation of a teacher. There is thus no conflict between the eternity of the Veda and the fact that the Rishis are the makers of its sections, hymns, and so on.
Also, as discussed in answer here Vedas themselves also confirm apaureshya nature:
Rcho akshare parame vyoman (1) Yasmin devA adhi visve nisheduh (2) yastan na veda kim rchA karishyati (3)
Riks exist in a supreme ether, imperishable and immutable (1), in which all the Gods are seated (2) ; One who knows not that, what shall he do with the RIk ? (3)
Adi Shankaracharya also in this section of Brahma Sutra bhasya states:
अतएव च नित्यत्वम् || 1|3|29||
AtaEva cha NityaTvam
- And from this very reason there follows the eternity of the Veda.
As the eternity of the Veda is founded on the absence of the remembrance of an agent only, a doubt with regard to it had been raised owing to the doctrine that the gods and other individuals have sprung from it. That doubt has been refuted in the preceding Sûtra.--The present Sûtra now confirms the, already established, eternity of the Veda. The eternity of the word of the Veda has to be assumed for this very reason, that the world with its definite (eternal) species, such as gods and so on, originates from it.--A mantra also ('By means of the sacrifice they followed the trace of speech; they found it dwelling in the rishis,' Rig-veda Samh. X, 71, 3) shows that the speech found (by the rishis) was permanent. --On this point Vedavyâsa also speaks as follows: 'Formerly the great rishis, being allowed to do so by Svayambhû, obtained, through their penance, the Vedas together with the itihâsas, which had been hidden at the end of the yuga.'
Adi Shankara in the previous Sutra gives very long reasoning about how mentioning of things like gods, species etc.. by Vedas which have origin and death also doesn't prove Vedas are uneternal:
- If it be said (that a contradiction will result) in respect of the word; we refute this objection on the ground that (the world) originates from the word, as is shown by perception and inference.
Let it then be granted that, from the admission of the corporeal individuality of the gods, no contradiction will result in the case of sacrificial works. Still a contradiction will result in respect of the 'word' (sabda).--How?--The authoritativeness of the Veda has been proved 'from its independence,' basing on the original (eternal) connection of the word with its sense ('the thing signified') 1. But now, although a divinity possessing corporeal individuality, such as admitted above, may, by means of its supernatural powers, be able to enjoy at the same time the oblations which form part of several sacrifices yet it will, on account of its very individuality, be subject to birth and death just as we men are, and hence, the eternal connexion of the eternal word with a non-eternal thing being destroyed, a contradiction will arise with regard to the authoritativeness proved to belong to the word of the Veda.
To this we reply that no such contradiction exists.--Why?--'On account of their origin from it.' For from that very same word of the Veda the world, with the gods and other beings, originates.--But--an objection will be raised--in Sûtra I, 1, 2 ('That whence there is the origin, &c. of this world') it has been proved that the world originates from Brahman; how then can it be said here that it originates from the word? And, moreover, even if the origin of the world from the word of the Veda be admitted, how is the contradiction in regard to the word removed thereby, inasmuch as the Vasus, the Rudras, the Âdityas, the Visvedevas, and the Maruts are non-eternal beings, because produced; and if they are non-eternal, what is there to preclude the non-eternality of the Vedic words Vasu, &c. designating them? For it is known from every-day life that only when the son of Devadatta is born, the name Yagñadatta is given to him (lit. made for him) 2. Hence we adhere to our opinion that a contradiction does arise with regard to the 'word.'
This objection we negative, on the ground that we observe the eternity of the connexion between such words as cow, and so on, and the things denoted by them. For, although the individuals of the (species denoted by the word) cow have an origin, their species does not have an origin, since of (the three categories) substances, qualities, and actions the individuals only originate, not the species. Now it is with the species that the words are connected, not with the individuals, which, as being infinite in number, are not capable of entering into that connexion. Hence, although the individuals do not originate, no contradiction arises in the case of words such as cow, and the like, since the species are eternal. Similarly, although individual gods are admitted to originate, there arises no contradiction in the case of such words as Vasu, and the like, since the species denoted by them are eternal. And that the gods, and so on, belong to different species, is to be concluded from the descriptions of their various personal appearance, such as given in the mantras, arthavâdas, &c. Terms such as 'Indra' rest on the connexion (of some particular being) with some particular place, analogously to terms such as 'army-leader;' hence, whoever occupies that particular place is called by that particular name.--The origination of the world from the 'word' is not to be understood in that sense, that the word constitutes the material cause of the world, as Brahman does; but while there exist the everlasting words, whose essence is the power of denotation in connexion with their eternal sense (i. e. the âkritis denoted), the accomplishment of such individual things as are capable of having those words applied to them is called an origination from those words.
How then is it known that the world originates from the word?--'From perception and inference.' Perception here denotes Scripture which, in order to be authoritative, is independent (of anything else). 'Inference' denotes Smriti which, in order to be authoritative, depends on something else (viz. Scripture). These two declare that creation is preceded by the word Thus a scriptural passage says, 'At the word these Pragâpati created the gods; at the words were poured out he created men; at the word drops he created the fathers; at the words through the filter he created the Soma cups; at the words the swift ones he created the stotra; at the words to all he created the sastra; at the word blessings he created the other beings.' And another passage says, 'He with his mind united himself with speech (i. e. the word of the Veda.--Bri. Up. I, 2, 4). Thus Scripture declares in different places that the word precedes the creation.--Smrti also delivers itself as follows, 'In the beginning a divine voice, eternal, without beginning or end, formed of the Vedas was uttered by Svayambhû, from which all activities proceeded. 'By the 'uttering' of the voice we have here to understand the starting of the oral tradition (of the Veda), because of a voice without beginning or end 'uttering' in any other sense cannot be predicated.--Again, we read, 'In the beginning Mahesvara shaped from the words of the Veda the names and forms of all beings and the procedure of all actions.' And again, 'The several names, actions, and conditions of all things he shaped in the beginning from the words of the Veda' (Manu I, 21). Moreover, we all know from observation that any one when setting about some thing which he wishes to accomplish first remembers the word denoting the thing, and after that sets to work. We therefore conclude that before the creation the Vedic words became manifest in the mind of Pragâpati the creator, and that after that he created the things corresponding to those words. Scripture also, where it says (Taitt. Brâ. II, 2, 4, 2) 'uttering bhûr he created the earth,' &c., shows that the worlds such as the earth, &c. became manifest, i.e. were created from the words bhûr, &c. which had become manifest in the mind (of Pragâpati).
Adi Shankara also while talking about beginningless nature of Samsara also quotes the same quote:
For if it had a beginning, the prânas would not exist before that beginning, and how then could the embodied Self be denoted, with reference to the time of the world's beginning, by a name which depends on the existence of those prânas. Nor can it be said that it is so designated with a view to its future relation to the prânas; it being a settled principle that a past relation, as being already existing, is of greater force than a mere future relation.--Moreover, we have the mantra, 'As the creator formerly devised (akalpaya) sun and moon (Rig Veda. Samh. X, 190, 3), which intimates the existence of former Kalpas. Smriti also declares the world to be without a beginning, 'Neither its form is known here, nor its end, nor its beginning, nor its support' (Bha. Gî. XV, 3). And the Purâna also declares that there is no measure of the past and the future Kalpas.
So, by all reasonings and scriptual quotations given in the commentary, it is proved that mentioning of such things doesn't make Veda uneternal. So, if Vedas mention receiving knowledge from grandfathers or ancient scholars, it means that same thing was also told in previous creations and same thing will also be told in future creations. ie. It is propounding cyclic events which happens in every cycle.