This is football and anything can happen. Although Mbappe is an important player, he is not an irreplaceable player for PSG. The Qatari president has the money to transfer a good player. He may be aiming to make transfers that will make Mbappe forgotten. You may see that they buy such a player this summer that everyone will be surprised and Mbappe's departure will be forgotten. There are also competitive teams for League 1 such as Monaco, Marseille and Lyon. With a few teams in better shape, the competition can rise to a higher level. Time will show that
Mbappe has been part of PSG's success for many years. Most of the wins PSG has achieved so far are thanks to Mbappe's hard work who always scores lots of goals, so it's not easy for PSG to forget their star player. Time does go on and PSG shouldn't dwell too much on sadness and must immediately move on, but that doesn't mean forgetting Mbappe's services which have contributed greatly. Mbappe will always be remembered by PSG even though his position has been replaced by another player. No matter how much money PSG spends to recruit good players, I doubt they can get a great player like Mbappe.
If PSG has bought the players they need then they don't need to worry about Mbappe leaving which will cause pain for the club, I know PSG's level in the league is indeed the strongest but that was when Mbappe was there but for next season I think other competitive clubs like the club what you mentioned will make it difficult for PSG in the league, it will be more interesting if they compete with each other in the fight for the title so that League 1 matches will be more interesting because their level will be on par with other competitive leagues.