Mbappé is still young, with a long career ahead of him. So if he performs aggressively from now then he can perform better later. Mbappe can give Real Madrid a good dream because until now the Real Madrid team has been strong enough, then the Real Madrid team has become even stronger due to the addition of Mbappe to the club. And these titles from the present to the future must be won.
Real Madrid is still able to win the UCL trophy even without Mbappe presence in the squad. I think your assumption is wrong if you say Mbappe can give Real Madrid beautiful dreams. In my opinion, on the contrary, Mbappe can realize his dream with Real Madrid of winning the UCL trophy, so far he has never felt the sensation of lifting the trophy.
Mbappe presence will make Real Madrid stronger, this cannot be denied based on his performance on the pitch. In the future, Real Madrid will be the club that has the greatest chance of winning every championship. Their challengers must be ready to build their squad to match the strength that Real Madrid has if they want to win the most prestigious trophy in the European championship.