Manchester United have got way richer than before certainly. But as long as Real Madrid present the same conditions on his contract I don't think Mbappe's choice would be the Red Devils. Because they haven't been as successful as Real Madrid in the recent years before anything else. Mbappe must be hungry for winning a Champions League title.
In addition, Real Madrid constantly monitors Mbappe's situation in PSG. If Mbappe has the intention to leave PSG, Real Madrid must be the first team to know it. So, Real Madrid is likely leading the competition to sign Mbappe.
To me it seems PSG is going to die and they losing the stars they have like Mbappe they already lose Messi like other teams like Real Madrid with a pocket full of money waiting for these players and we already saw Mbappe rejected to offer to extend his contract, that's why it's pretty much normal to see him leaving his team soon.
PSG also wants him to either extend or they will sell him, no intention of sending him away for free, so either he extends or he will be sold, that's also understood. But on this case, what could PSG do if he doesn't leave? I mean if he refuses to leave, then there isn't much they can do about him, just bench him all year long? I do not think so, fans would be mad, the whole world would question that. If he wants to stay one more year, PSG will have to stuck it up and just let him play for another year and then leave.
Well its good for PSG that he is staying,a lot of players are leaving PSG and if he leaves they would be really bad next season. They still need to sign a lot of good players if they want to be an elite team like before and with Mbappe they now have hopes and can build a team around him for next season.
For a team like PSG with many famous players, this can be a pain to replace the players and find alternative players for stars like Mbappe because even if they start seeking to find players like him still they have to pay even more money to hire these players.