Perished, or people simply migrated?1) Yes, but it didn't survive. Pre-vedic civilization perished.
2) On the contrary, Aryans were smaller in numbers. But this point is inconsequential, because pre-vedic civilization was already dead.
Latest studies claim that they migrated due to climate change, towards Gangatic plains.
Wrong. South Asia had seen invasion/migration of Kushans, Greeks, Persians...since BC and early AD era. Muslim invasion only came about after 900 AD.
India was never isolated, it was a crossroad between middle east, central asian and far east China.There was strong persia influence. Indo-greek empire had significant impact too.
Compare to the interaction within India, those interactions were always limited. So much that locals considered people outside of subcontinent as foreigners and called them with a sort of derogatory term: Mleccha - Wikipedia, the free encyclopedia .
Significant influence of outside world only came after Muslim invasions.
Indus civilization is not entirely a dravidian one, there was significant evidence of Sumerian influence. In any case, it is a moot point, the civilization was dead. What you have now is foreign vedic civilization.
The civilization died of climate changes, not of human causes. The end of the period is also coinciding with so called Aryan migration. Doesn't changes the fact that indigenous people were well capable of forming a civilization, which they did again.
And the vedic civilization was created within India, don't see how this is foreign.