I'm not so sure why people think Phase II wouldn't have been successful, TNG essentially is Phase II with the names changed. I think Star Trek was popular enough that Phase II would have been OK, but probably wouldn't have lasted more than three or four seasons as that was more common duration for shows of the time. No 80s movies, but there probably would still have been a TNG-like TV show in the late 80s, since a lot of older TV properties were being rebooted or having new continuations around that time (although, i suppose some of those shows were probably greenlighted due to TNG's success). However, I think such a TNG show would actually have been the "next generation" using older Phase II characters and not 100 years in the future.