PSG will not be same like PSG that people know except they can buyout Haaland lol.
I expect there will be declines in Ligue 1 where people not really interested to watch the match and perhaps less discussion in this thread too. Mbappe is like Ronaldo, he's the one who bring the crowd, not the league or the team.
In addition, during this time Mbappe actually developed when PSG dominated Ligue 1 and if Mbappe was still and playing for Monaco of course we might never hear his name as a star player, in my opinion, when PSG can dominate Ligue 1 then automatically Mbappe's performance and name are raised to the surface because indeed Mbappe has been getting support from other star players in making him the top scorer.
Losing mbappe of course will not make Ligue 1 lose its prestige in the future, especially now that PSG has several names that are targeted to be replacements for mbappe next season so I think gradually fans will be able to forget mbappe later, what you said has actually been said by many people when messi and ronaldo left La Liga, But is it now without Messi and Ronaldo that La Liga has lost its prestige?
Of course not because indeed players will come and go when the time comes and of course this is also inevitable in Ligue 1.