this wholesale acceptance of a culture with no traces of an earlier culture having been replaced, nor remembered has almost no parallel & certainly not on this scale.
In two words: caste system.
Let me expand. A group of invaders (the "Aryans") came from the North West and, by means unspecified as yet, gained control of the ruling/priestly classes in Northern India. The means could have been military conquest, marriage, bribery, philosophical debate, whatever. That's an unknown, and a big hole, but let's leave that for now.
Once these invaders controlled the priesthood, they instituted a strict social hierarchy which would later evolve into the full blown caste system. The reason for this hierarchy was two-fold:
a) to keep their own "Aryan" race pure, and
b) to have strict control over all trans-generational cultural transmission. The masses were forbidden to know anything other than what the priesthood provided. It was a means of cultural genocide, pure and simple.
This had the effect of minimizing the genetic impact of/to the invaders and also wiping out any memory of indigenous culture. The Dravidian ruling elite were all for it since it ensured them an intellectually enslaved and docile populace. This appeal to the rulers may have helped spread this new ideology throughout North India and, later, beyond.
The Rg veda already speaks of fighting between clans (no castes yet) while by the time of the Mahabharata, castes are an established phenomenon.
Sure. The hierarchy I postulated above was the proto-caste system in place during the time of the Rg Veda. It only had three roles: Dravidian rulers, Aryan priesthood, and Dravidian masses. Over the centuries, this evolved into the full blown caste system where the Dravidian natives were incorporated into the priestly Brahmin classes, fully diluting the Aryan genes into the local gene pool. Also, by this time, the Aryan-imported Vedic culture had become the native culture. Indeed, the only culture.
Yet the Mahabharata enlists distant Aryan tribes from Afghanistan/Iran on the basis of kinship (which should have been long diluted & in case should not have been responded by parts of the tribes that were not incorporated into a local "Indian/Indus" population) suggesting an unbroken feeling of kinship between the diluted & the non-diluted. The other alternative would be to suggest that the distant Aryan tribes too were diluted similarly, yet still had an almost identical reaction of not remembering being any other people. I suppose possible theoretically but quite a stretch.
No issue here. The Aryan priesthood knew exactly where they had come from and whom they were related to. The Dravidian masses had been indoctrinated over so many generations now that they would chant anything.
You could push the dating of the events mentioned in the Mahabharata as far back as you want, it is just that it takes the Rg veda further back & that does not fit in with the AIT as propounded now.
I am not trying to push anything back. My earlier comment applies equally well to the Vedas. Barring archaeological or other independent evidence, there is no reason why any religious text should be taken at face value to refer to actual contemporary events. Even the oldest text could simply be retelling even older oral traditions and myths.