I can imagine a bunch of Americans getting the LeBron Olympiakos jersey just because it's different. Their merch sales would go through the roof.
As far as the Cosmos and the
NASL, they sold out Giants Stadium once, and while drew a good number of fans, having the best player in the world helped with that. There is no danger of the Euroleague shutting its doors because they are throwing money at high priced Americans.
As far as the original question, I don't see the NBA falling that far off. Guys like LeBron will look for the quick payoff, but will eventually make their way back to the NBA.