Away from the joke that the NFL is becoming... in case you haven't already seen the news, attendance is already down.
There are a lot of "football" sports around the world -- soccer, rugby union, rugby league; American football, Canadian football, Celtic football.
American football certainly isn't the worst or the least interesting of them all.
A sport's popularity isn't written in stone, but the NFL could make it a little softer or duller without the home viewing audience deserting.
Given the fate of the USFL and other new leagues, I don't think you're going to see a competing league displace the NFL either.