Ancient Acharyas say that it is only logical to accept the validity of Smriti, because the expounders of Shruti also expound the Smriti. Hence, the Smriti is based on Shruti.
Firstly, we accept the Vedas as eternal and divine because Vedic scholars say so. In other words, we have trust in Vedic scholars and the Vedic Sampradayas that produce very learned, trustworthy, and religious Vedic scholars. So when those Vedic scholars say, "The Vedas are divine and eternal," we believe them.
The expounders of the Vedas are the basis for the trustworthiness of the Vedas. This is the basis for the trustworthiness of the Vedas.
Now there might be some sectarian differences, like Nyayas believe the Vedas were divinely composed by Ishvara. Some others say the Vedic swaras are eternal, but the Rishis composed the words of the Vedas.
Either way, what almost all ancient Vedic sampradayas believe is that the Vedas are eternal and the primary authority for spiritual matters.
Having established that the Vedas are the foremost authority because they are eternal, what about Smritis which are authored? Do Smritis have any authority?
This question is answered in the ancient, very important work called the Purva Mimamsa Sutras, which were authored by Jaimini Rishi, who was a sishya of Vedavyasa.
1.3.1 - [Purvapaksha] In as much as Dharma is based upon the
Veda, what is not Veda should be disregarded.
1.3.2 - [Siddhanta] But Smriti is trustworthy, as there would be inference (assumption, of the basis in the Veda) from the fact of
the agent being the same.
1.3.3 - When there is conflict between Shruti and Smriti, the Smriti should be disregarded; because it is only when there is no such
conflict that there is an inference of Vedic text in support of
Sutra 1 is the opponent's viewpoint. The opponent is saying that everything that is not a Vedic text should be disregarded because only the Veda is the source of Dharma.
Sutra 2 is challenging that and saying that the memory of the expounders of the Vedas is also a Pramana. This is the position of Jaimini himself. He wrote this Sutra. There were thousands of Vedic shakhas in existence in ancient times, and only a handful have survived today. The Dharma Shastras are a summary of the topics of particular Vedic Shakhas, and the Smriti is the recollection of the expounders of that Shakha.
In other words, at one point, there may have been a Vedic shakha that enjoined certain things not existing in any extant shakha, but the Vedic text that enjoined that practice was lost, and the only thing that is left is the "memory" of the expounders of the Vedas. This is what is meant by Smriti.
Basically, Sutra 2 is saying just as the Parampara expounds the Vedas, they also expound the recollections that were presumably once based on the Vedas.
So, it is illogical to believe Vedic scholars when they say "This is a Vedic text," and not believe them when they say "This is a Vedic injunction, but I have lost the text."
And it is especially illogical to accuse them of fabricating verses, because then you can just as well accuse them of tampering with the Vedic texts.
And finally, Sutra 3 just says that when there is a conflict between Shruti and Smriti, discard Smriti and go with Shruti, because the inference of a lost Vedic text to corroborate a Smriti verse is only valid when there is no conflict between Shruti and Smriti. In other words, since Smriti presupposes Shruti, if an extant Shruti text refutes a Smriti text, then the Shruti text is more authoritative since it is direct perception of a Vedic passage, and direct perception is always stronger than inference.
Manu also lists the 4 sources of Dharma:
2.6 - The entire Veda is the root-source of Dharma; also the
Conscientious Recollection of righteous persons versed in the Veda,
the Practice of Good (and learned) Men, and their self-satisfaction.
"The entire Veda" is Shruti.
"Recollection of righteous persons" is Smriti.
"Practice of Good and learned Men" is Achara.
"Their self-satisfaction" is Atmanastushti.
Also, the Vedas themselves say that Smriti is valid.
The Chhandogya Upanishad itself says that the Puranas and Itihasas are the 5th Veda:
7.1.2 - Nârada said: 'I know the Rig-veda, Sir, the Yagur-veda, the Sâma-veda, as the fourth the Âtharvana, as the fifth the
Itihâsa-purâna (itihāsapurāṇaṃ pañcamaṃ vedānāṃ)
Same with the Brihadaranyaka Upanishad, which says that even the Smriti was inspired by Brahman:
2.4.10 - 'As clouds of smoke proceed by themselves out of a lighted fire
kindled with damp fuel, thus, verily, O Maitreyî, has been breathed
forth from this great Being what we have as Rig-veda, Yagur-veda,
Sama-veda, Atharvâṅgirasas, Itihâsa (legends), Purâna (cosmogonies),
Vidyâ (knowledge), the Upanishads, Slokas (verses), Sûtras (prose
rules), Anuvyâkhyânas (glosses), Vyâkhyânas (commentaries) 1. From him
alone all these were breathed forth.
The Taittiriya Samhita of the Krishna Yajur Veda says that the Manusmriti is authoritative:
Whatever Manu has said is beneficial.
And finally, quotes from various Dharma Shastras:
Baudhāyana, Dharmasūtra, 1.1.6.—‘Dharma has been enjoined in each
Veda.’ ‘The second source of knowledge consists of the Smṛtis.’ ‘The
third is what proceeds from the cultured, i. e., those persons who are
free from jealousy and selfishness, fairly well off, free from
avarice, haughtiness, greed, delusion, and anger.’ ‘Those persons are
cultured who have studied the Vedas along with their supplements and
who are versed in the art of making deductions from them; those are
the persons from whom the direct knowledge of Śruti can be derived.’
Gautama, Dharmasūtra, 1.1-2.—‘Veda is the source of Dharma’: ‘the
Smṛti and Śīla of persons learned in the Veda.’
Āpastamba, Dharmasūtra, 1.2-3.—‘The convention of persons knowing
Dharma is authoritative’: ‘and also the Vedas.’
Vaśiṣṭha, Smrti, 1.4-6.—‘In the absence of Śruti and Smṛti, the custom
of the cultured is authoritative’: ‘those persons are cultured whose
mind is free from selfish desires’: ‘that is to be regarded as Dharma
which is not prompted by a selfish motive.’
Yajñavalkya, Smṛti, 1-7.—‘Śruti, Smṛti, the practice of good men,
self-satisfaction determination based upon right volition,—these four
are the source of Dharma.’
Āpastamba, Dharmasūtra, 1.4.7.—‘The Śruti is more authoritative than
custom which derives its authority only from assumption (of
corroboration of Śruti).’
Ibid, 30.9.—‘In cases of conflict, what is stated in the Śruti is more
Therefore, it stands that all ancient Acharyas accept the authority of Smriti, and that there is a logical foundation for accepting both Smriti and Shruti.