Ronaldo has already said he will stay at United even if they fail to qualify for the Champions League. United is more than just another club for him, so I don't think he will change his mind. Given his age, there is a possibility that we will not see Ronaldo in the Champions League anymore, but I agree with his decision. Let him finish his "big" career at United, and in retirement it will be possible to play in America or somewhere else.
I do not think that he is the type of player that will to MLS to play a few years and earn a ton of money. Don't get me wrong he would beast there even at age 40 for sure there is no doubt about that, but I can see him more like Lisbon type of player who will finish it where he started at Sporting Lisbon by not playing that badly but not providing anything of worth neither but that is a few years away, not anytime soon. He will probably offer something decent to United for another 2-3 years at least.
If we just take a time and consider what that means, he would be actually providing something valuable to United, a team at top 10 of premier league, at striker position at around age 40, that will be insane.