Messi wasn't the reason they won so many titles before he arrived, they got titles before him and would get one after him as well. But Mbappe is certainly a question mark, we need to see what the teams decision will be, there is a chance he may get sold off, and he doesn't want that unless team agrees to pay a huge amount of money, and team wouldn't want that but we are going to see what's going to happen with him.
I think the better move for his career could be going to Madrid but for some reason or another he just doesn't do it this season, he insists on going there next season and I am not entirely sure why "money" means so much when we all know he ain't getting stiffed, Madrid would pay him too, not like he is giving up money, he will be paid.
Yes, the departure of Messi can rather be considered a relief for PSG. By the way, like the departure of Neymar, but there is still no certainty with him.
As for Mbappe, in my opinion everything has already been decided, but now there is a bidding process Real wants to get Mbappe for less than 250 million (quite reasonable given the remaining year on the contract), PSG, on the contrary, is trying to get as much as possible, like Mbappe who pretends to be ready play another year at PSG.