Well, this is a completely possible scenario, and it seems to me that it was discussed somewhere whether Mbappe will remain at PSG if Enrique suddenly manages to win the Champions League. But I agree that Mbappe will most likely want to leave for Real even in this case, because this is a long-standing desire, a point that needs to be fulfilled.
Mbappe didn't sound like he was leaving PSG to Madrid because of the inability of PSG to been able to win the champions league so if fortunately PSG gets to win the champions league this season I doubt if that will be enough reason for mabappe to stay back because PSG still wants him and would pay him some more if he wanted but that doesn't seem to be like the case with PSG a d mbappe.
If PSG wins the champions league it will only be a plus to his career adding to the fact that he's got a champions league at PSG and it will even be more of a glory to him in that he won the champions league before leaving the team so for mbappe, I think it's actually beyond the quest to win the champions league at PSG and so that will not be a good reason enough for him to stay back because he's decided to already move without conditions . Mbappe wants to probably grow his career and I think real Madrid isn't a bad place at all because there he's very likely to get all that he's desired in he's career so far so it isn't a bad idea for a place to further his career.
In Madrid he will aswell be having a whole lot of play time because he's position where he will be playing currently has got no one occupying it which he would be contending the position with so he's gat to play well although if he happens to be injured or have a reason not to play at any time there's a strategy with which they could make up for his absence, I think Madrid will be such a great deal in the career of mbappe.