I dunno, if you're an adult who moves from India to America, you're not going to be speaking Hindi much (unless you speak it at home, to be fair). You'll be conducting your life entirely in English. And if you move to the States for university then you'll be doing academics in English too. You'll easily achieve thousands of hours of practice in this way.

While most people don't have time to put that many hours into studying a second language outside of a country where it's the native tongue, many immigrants have no choice but to immerse themselves in a new language just by the default behavior of living their life in a new country that doesn't speak their first language.

Yes, such immigrants will always be a few thousand hours behind people who started in America, but surely there's some plateau where the improvements become marginal. I don't think the critical period advantage is about the early volume of hours so much as the age period when they're spent - i.e. if you compare someone who moves at 20 vs. 40, the results will be the same, but both will be worse than someone who moved at 8.