If we talk about Ligue 1 then we cannot deny that PSG will still dominate Ligue 1 even without Mbappe in their current squad, but I think for now it is not the Ligue 1 trophy that is their main priority to get but the UCL trophy for the first time , so this requires PSG and Enrique to evaluate the strength and quality of the team they currently have, the results of the friendly match against Sturm Graz yesterday should be a reference for Enrique to see the weaknesses that PSG currently has, competing in the UCL is not as easy like as in the Ligue 1, so like it or not Enrique must to maximize and improve the current squad.
I also agree more with what you said because Enrique must be able to maximize the strength of the team he is coaching in order to compete fiercely in the UCL and get his first trophy in the competition. Because PSG still often fails to win the UCL during his career, Enrique still has to be able to give his best for PSG with the support of the best players he can rely on this season and can continue to focus on big competitions like the UCL because in Ligue 1 PSG will still be a very dominant team to win.
While he does need to make the team better, it also means that he can't do it all by himself, he needs help from the staff, from the management and from the players as well. If the players give up because they think they can never be as good without mbappe, then they will not provide 100% of themselves to the games and Luiz Enrique can't do much about that, as a manager he is responsible for making players play better, but after certain point no matter who you are, pep or ancelotti or enrique it doesn't matter, players who don't want to play will not play.
Whereas, if management doesn't bring him the players he wants, doesn't matter if they get great players, sometimes even the best player will not be a good fit, so they need to listen to the manager when they are making transfers, we don't know if he requested these players or not, but if he did then the management did what they suppose to do.
I still expect them to be fine, but people should not expect much from them on UCL, just going beyond the group stage is the most expected thing, that's it, nothing more could happen for them at all. We are going see them just be decent at the league, and even there they will not dominate.