Paris Saint-Germain had even Lionel Messi, Kylian Mbappe and Neymar in addition to other names in the same team few years ago, yet they failed to bring UEFA Champions League home. It is only about how many stars you have in the squad but about how good and coherent they are.
The actual team doesn't have exceptional players, they are all skilled and playing under a good coach at the time where their opponents were suffering from horrific instability without exceptions. That consistency of PSG and the instability of the other teams guaranteed the success of the team.
They were smartly by letting Neymar and Messi go away when Neymar was very sensitive with injuries and Messi was old enough with declining performances. They only tried to convince Mbappe to stay in their club but after many attempts, they wisely let Mbappe go.
After that PSG have entirely changed their club development strategies that were proven as good decision with this UEFA Champions League title. Throughout this season, they already built up a more balanced team than past seasons and there are discoveries in this season like Barcola and Doue who are very promising young players for PSG in future.
So far, I believe that PSG have no reasons of regret about departure of Mbappe as Doue and Barcola have potentiality of becoming new Mbappe for PSG in coming years.