If in the end there is a deal, I'm pretty sure if he can bring PSG much better than Galtier. but there is one problem, can Nagelsmann have full control in the dressing room. if so, I am optimistic that PSG will be able to bounce back. regarding players, they can bring in anyone after all PSG has abundant finances. The problem is, so far no coach has been able to bring PSG to lift the Champions League trophy. maybe with Nagelsmann and a series of new players brought in, PSG's lucky season will come. but by the way, this is just a personal opinion, even the league competition just ended. so, we'll see how it turns out later.
However, he is not a tactical genius which means that he is not going to be all that great after like 6 months, when all the transfers are done and its the dog days of the year, he is not going to be able to adjust his system to whatever team needs and that's when they will realize it wasn't a great idea. He is the anti-ancelotti, ancelotti takes a team that's ready to its peak ever possible unlike anyone else, but doesn't usually build it himself.