They haven’t saturated, the fact that teams move proves that. But there’s 6.7 billion non-Americans in the world. An international footprint is good for sports, more money, more talent.
Teams move because there’s always a city out there willing to pony up for a new stadium. From what I’ve seen, some NFL teams that never had trouble selling out games are now playing in front of some empty seats.
I would think the NBA — which plays a game that already has proven to be very popular in Europe — would do this long before the NFL does.