Messi is the best footballer ever, but he is past his prime as well. He did well by a regular star’s standards at PSG these past few years so of course he would wreck MLS defenses, but he was nowhere near the Messi of even a couple of years ago (part of this was PSG’s system).
There are many factors why Europeans would have little interest in watching the MLS, even with Messi in it, just like they’re not watching the Saudi league because Ronaldo is there and Benzema just joined as well.
There is so much high level football played at the national and continental level being played in Europe, and superstars like Haaland and Mbappe are firmly taking their place as the faces of football in Europe, that unless you are a Messi or Ronaldo specific fan, the average football follower would not go out of his way to watch either of them in leagues in other continents.
As far as money goes MLS just needs to be as big as MLB is today to afford premier league salaries - it doesn’t need to hit NFL levels of success.
MLS teams have much much larger potential local fan-bases. In MLS territory (US + Canada) there are about 370 million people. In Premier League territory (England + Wales) there are approx 59 million people.
So theoretically you can easily be the a secondary sport in the US and still be the best in the world (see NBA).
But still I don’t think we’ll knock out Premier League anytime soon. Too much tradition to be taken out quickly.