We really don't know how a team like PSG will develop next season and in the seasons to come, but what you said is actually enough of an example that a team can still rise with new players recruited by its coach into the squad. And this could also happen to PSG in the coming seasons because the good players are not only Mbappe, but there are also many other seeds that the team can use for this. PSG fans can only hope for the best for PSG next season by no longer having to depend on just one player.
In fact, when Ronaldo was at Real Madrid and Messi at Barcelona they were basically old, so the team could regenerate players. But regarding Mbappe this is different, because Mbappe is still young and PSG is also not regenerating players and thus, losing Mbappe will really have a big impact on PSG's performance. After all, when Ronaldo left Real Madrid, Real Madrid still had other great players, which made Benzema shine after Ronaldo left. However, regarding Messi at Barcelona, as far as I remember, Barcelona did not regenerate players, so their performance continued to get worse, especially regarding their financial problems.
So yes, I think this case has differences and cannot be compared between them, because even though PSG lost Mbappe, but PSG is a team that has good finances. But despite this, it is still difficult for PSG to improve the quality of the team, especially to compete in the Champions League. But yes, because Mbappe has a big influence on the success of the PSG team, then with Mbappe departure will also make it seem like it will be difficult for PSG to dominate Ligue 1 like before.
I don’t particularly think that without Mbappe, PSG will face hardships. At least not in Ligue 1. Football is not played by 1vs11, rather it's a team game. Yes, it can be argued that Barca became arguably worse after Messi's departure but they weren't really that good when he left.
If you talk about PSG, they have good finances, a solid coach, and a decent amount of players in the club still. If they play their cards right, they can still dominate the Ligue 1.
About the UCL, Luis Enrique would need good reinforcements, as they couldn’t win it even with Mbappe. He needs to sign a proper striker and then need to balance out the whole squad, and if they do that, they might do well in the knockouts.