Relegation and promotion system makes soccer more fun. It won’t happen in America any time soon because MLS is what it is today because team owners dumped a bunch of money into it, which I understand as they don’t want to risk losing money. However, relegation system incentivizes competition way more and allows for more areas of the country to have a pro team, which ultimately would help the sport grow.
Regarding Messi. Dude is absolutely gonna dominate at the MLS level.
Most fans of for example PL football don’t give a **** about promotion/relegation because the majority of people support Man U, Man City, Liverpool, Arsenal, Spurs, etc. I’m an Arsenal fan, there’s nothing exciting or not exciting about promotion/relegation because I don’t even think about it because it doesn’t affect my club.