It would be a bad decision from Mbappe because PSG are moving closer to Champions League title. They only need to keep up their good works and a bit more luckiness in their matches to win Champions League.
Real Madrid now are in the early of a new generation and Mbappe will join a club that is similar to PSG a few seasons ago. I think Mbappe will get all he want in PSG. Just a bit more time, a few more seasons. Mbappe is one of most important players in PSG and he will get same position in Real Madrid but without big difference in his position, he should not leave PSG.
Most times, pumping money into players doesn't guarantee champion's League success. Manchester City and PSG have pumped millions for years into buying world-class players, but look what they have achieved in the UCL so far. Nothing!
Meanwhile, on an average season, Real Madrid can still win the UCL.