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.
It is great for Manchester United in that case. Because I'm pretty sure that they wouldn't like to see Ronaldo leaving already now. His contribution to the team during this season has been just incredible and he would still continue to do this next season also. Manchester United still had better add a top striker to the team now but Ronaldo wouldn't be in the subs then of course. They would find a formula for it not to create a problem suddenly. Ronaldo must be enjoying to play here then by the way.
According to updated data (rumors

), everything has changed and now Ronlado is likely to move to some other club (even the possibility of moving to PSG is being discussed). I would be happy if he stayed at United, but his move to some big club (PSG or City, both of which I don't like) would be the highlight of the transfer market and a big scandal.