I'm not sure what you mean? The MLS is barely acknowledged over here in any capacity; not being rude or dismissive, but you'd really struggle to find many with any knowledge about the league whatsoever.
Even if he went there and did miraculous things, there'd be no frame of reference and barely any tangibility as opposed to him really making a name for himself getting a barely functional club to play some of the best football in that league, and by extension, the country (even if you added all PL teams to that, he'd be nestled in top 20-35 in the country, but you can remove a host of PL clubs immediately and that gives a more rounded idea of how impressive the football he had them playing actually was).
Some on here may have scoffed if he was given a PL side as his next gig whilst others would be genuinely interests to see what he could do given what he managed with an anti-squad. All that is out of the window if he goes to the MLS for any concerted period of time; he might even have to start again at Championship level to prove he still has it, even.
It's not even a sideways step as it pushes him backward.
He's a very rich man and doesn't have to follow the generic career trajectory path, which is where he gets an 'ah, it's probably for personal reasons' whereas any normal up and coming manager would have more questions asking what the hell kind of career move is that? Than Rooney will ever have.