It would be an interesting experiment. I would guess that it’s that shot at a championship that makes the major leagues more interesting. That is, if you know at the beginning of the season that your team has no chance at a championship, that’s going to reduce the appeal of the season. That means less people in the stands, which means less money, which puts us back at the objection to relegation.
Put another way, it’s like if the playoff teams were decided on opening day. How interested would fans continue to be if they know from day one that their team doesn’t have a chance? I honestly don’t know. It seems to work somewhat in college sports. I just don’t know if it would work in the majors in the US.
I think MLS is probably on track to be the relegation league, with the top teams going to the Premier league. It seems inevitable that the US will get a Premier league team, and graduating MLS teams seems to be the best way to do it. My completely unscientific and uniformed guess. ;-)