Maybe this is still in context and honestly I am happy to see Mbappe win his first European title elsewhere.
What would have happened if Mbappe had stayed at PSG? Of course it is impossible for him to win a European title and with this Supercup title, one of many to come will be created. We can see from the hundreds of matches with PSG that he has never gotten what he has wanted for a long time and once he played for another club in his first match he immediately got his dream.
The right decision from Mbappe by joining the Spanish giants and what would have happened if he had stayed at PSG for years and only chased the money, maybe he would have grown forever as a player with a large income and minimal European level trophies.
Mbappe has made a good choice, if he decided to go in the previous season then he could have gotten the Champions League title. A little late but this is not a problem because he is here. His career as a footballer is very bright, especially now he is already in a club that dominates the Champions League with 15 titles they won. I think that in the near future he will also get another pretension (near what I mean is 1 or 2 seasons at Real Madrid).