I have nothing but admiration to ISRO for what they have achieved
My only problem is what you wrote:
If anything, the brain drain consists of India's best and brightest relocating to the US after having studied in the near-free subsidized taxpayer funded colleges and universities in India. They typically go to the US to do their masters and PhD and then some of them join NASA
I do have business dealing with India, and I do regularly meet my business acquaintances in India, and I know, for a fact --- that so-called brain drain has actually benefited India
How, you might ask?
Those who emigrated abroad and working in America / Europe do accumulate knowledge / experience in whatever they do there, and, --- this is very important --- when they go back home, for visit (many of them still have families back home), most of them do pay courtesy calls to their own friends working inside India, and when that happened, technology transfers happen, and that event happens very very frequently in India
Many of the "tricks of the trade" that have been picked up by the Indian industry (including ISRO) came from the courtesy call from their friends / former classmate who work abroad
Of course, what I have described above isn't limited only to India, the same thing happens in Korea, Japan, Taiwan, and even in Russia and China as well
The "brain drain" in a way, do offer a "reverse channel" that enables knowledge to flow back to home countries