If Mbappe leaves the club in summer, which I think will happen no matter what PSG will do, the club will lose so much besides an exceptional striker. I think the whole team will suffer from Mbappe leaving them. They are so much less dangerous without him. Imagine you take him out of the team now, I think they wouldn't have the slightest chance in the Champions League as he is the player who can really decide a game within a blink of an eye. They would still win Ligue 1 as this is nothing special given how much quality they have in the team, but on the highest level Mbappe can't be replaced.
But I wonder, even though PSG has good finances or is the richest team in France but yes, it seems that PSG is not so successful in having and or getting young players with great potential. Because anyway, over time also young players with great potential like Mbappe will also leave and if Mbappe really leaves, then of course PSG will experience a decline in performance which may be very bad.
Because after all, so far PSG has only relied on Mbappe to become a goal machine and of course, if Mbappe leaves, it will take more time for PSG to get or have good performance only in Ligue 1. Because yes, we cannot consider too far about PSG in the Champions League because after all, only for competition in Ligue 1 also PSG still relies on Mbappe.
But think about it, if you were around 10 years of age and you are extremely talented in football, you have your idols and you are dreaming of certain leagues where you would love to one day play, would Ligue 1 be part of your dreams? This is the biggest problem I think for clubs from France. PSG is an exception because they can make up for a bad experience with money. Playing in Spain or England is very different from playing in France. El Clasico in Spain, the derbies in England and so many other things that are great in those leagues. The only thing that could convince you to go to France is money. No young talent would ever go to France if clubs from Spain and England are also interested.