It may be that Messi really does draw huge numbers of people, some of whom will become real fans, or it may be that the Messi crowds see all the mediocrity around him and decide to stick with whatever they were watching before.

I can't see how it would be anything other than option 2.

Messi is 36, he no longer can do the flashy dribbling he did at 20. Those are the things that won him Ballons D'Or and that built up his reputation. Even at his peak he spent a lot of his time walking around, not running. His sudden bursts of pace were exciting, but he was never really an "athlete". The only thing he really still has left is his incredible touch and his ability to read the game. But, going against that will be that in every MLS game he's going to have a much younger, much more athletic player stuck to him like glue. At PSG he was heavily covered, but his teammates were also a threat, so they couldn't just focus on him. In MLS he's it. So he'll be smothered by 1 defender all game, maybe 2 of them. His time on the ball will be very limited. So, he'll mainly have to be a passer of the ball from a range well beyond the goal. Passes aren't nearly as flashy as dribbles or goals, and they're harder for people who aren't big fans to appreciate. In addition, a pass requires that you have a teammate to receive the ball. At PSG and Barcelona he had elite teammates who would make clever runs. In MLS he'll have mediocre teammates with a poor first touch, poor reading of the game, and mediocre bursts of pace.

People who are knowledgeable will see a Messi in decline who is wasting his final years in a league that's beneath his abilities. People who aren't knowledgeable will see a player who isn't living up to the hype, who maybe occasionally shows up in highlight reels for a great pass or goal, but who mostly stands around or walks. Not worth spending 90 minutes watching.

As for MLS competing with other leagues, in reality it shouldn't compete with the NHL, or NBA because they're largely winter sports. It will have some overlap with the NFL, but it's an autumn / early winter sport. It's really only baseball that fully shares a season with MLS. It also shares a season with car racing, etc. but I think that's a pretty different audience. You would think that sports broadcasters would be eager for something to provide content for the time in the year when NFL, NBA and NHL were all off.