I just wanted to add my opinion as an answer. In regards to context, Krishna might have said so to stop Bramhanas and Ksatriyas from doing Materialistic Jobs, for their duties and responsibilities towards society are much higher. It doesn't use that harsh language on Vaishyas. As regards respect is considered, we can look at Vedas -
Salutations and salutations, To him who is the potter, And to him who
is the black smith. Salutations and salutations, To him who is the
hunter of birds who uses nets, And to him who is the fisherman.
Salutations and salutations, To him who hunts animals, And to him who
drags dogs using a rope. Salutations and salutations, To him who is a
dog, And to him who protects the dogs. (Rudram Chamakam 4.2.5-9)
O God grant love to our holy priests, set love in our ruling chiefs.
Grant love to the Vaishyas and Shudras; give out of Thy unbounded
store of love, love unto me!(Yajur Veda 18.48)
May all beings look upon me with the eye of a friend. May I see all
beings through the eye of friendship - may we all look upon each other
as friends. (Yajur Veda 36.18)
As regards Bhagavata also we can see another statement which asks us to understand that Brahman alone is pervading in all beings-
One should give up the bodily conception and should offer obeisances
before all- even the dogs,cows and asses falling flat upon the ground
like a rod. Until one has fully developed the ability to see Me within
all living beings, one must continue to worship Me by this process
with the activities of his speech, mind and body.(Srimad Bhagavatam
11.29.17)