So far he has fulfilled his dream of playing for his favorite club as a child and not long after he was at Madrid, Mbappe has won a trophy which will clearly be very difficult for him to get if he continues with PSG
He has not only chance to play for a club he loves but also has opportunity to lift UEFA Super Cup which was impossible for him to lift with PSG and many years with that club. It's good official start for Mbappe in Real Madrid, a goal and a trophy and onward he will move with bigger targets in this season.
He could have stayed another 10 seasons with PSG and would still not lift that cup, which is one of the reasons why he left. I mean he left because he loves Real Madrid, even if they weren't doing this good he would have still left because he loves them, but they are also amazing so it worked out amazing for him.
I bet that as long as he stays here for a while, we are going to see him win many more cups, from la liga to copa del rey to UCL to super cup, he is going to get a lot of them. If he plays another 10 or so seasons here, and 10 seasons isn't really unlikely for a player at his calibre, then he could have like 30+ cups by the end of his career, something very rare and not a lot of players can say. He came here relatively young, so he has time to grow with everyone else around him and do big things. I also think that he is going to keep scoring, and that magical 800+ goal mark isn't impossible for him, very very hard, but not impossible to achieve.