Based on personal experience (friends who either entered academia/industry or me being the son of white collar immigrants to the US) I'd say large portion want to return to China and India due to family ties, but a number would have stayed in the US if immigration wasn't such a PITA - especially because companies and programs don't really like filing for an O-1s unless they found a unicorn.

You hit the nail on the head on developing countries not being as poor anymore, and opportunties proliferating which reduces the pull factor, but there are a decent amount of academics and professionals who would gladly work in the US if given the opportunity and it wasn't such a headache.