The evidence is the number that actually become US citizens. Do they get credit for the uncovered years... don’t think so.
1. I don't think they get any credit for uncovered years.
2. Even if they DO become citizens, it's very difficult to get "extended family" (especially OLDER family members) in the country legally. I know a legal immigrant who is now a U.S. citizen who has given up on getting his mother moved here to the U.S. after trying for years.
3. This thread is filled with ignorant posts from people who apparently don't even understand what the visas listed here are. They are non-immigrant visas, and the FICA tax issue is irrelevant for some of these visas because someone living here temporarily under one of those visas isn't even allowed to work.