I believe PSG owners will give Mbappe a lucrative contract that will make him remain at PSG for some years. PSG has spent so much money on players to win the UEFA Champions League, but they have failed to do that. But I believe if PSG owners sign Zinedine Zidane, he will guide them to Uefa Champions League glory.
and everything you say has actually been answered yourself, that even if you keep Mbappe or spend a lot of money to buy star players, it still won't change PSG future to be more beautiful unless PSG management is committed to retaining the coach for several seasons and not always changing the coach quickly.
so the problem is not just who the PSG coach is, but the real problem is that PSG management often does not give the coach a chance when they fail to win the UCL title, whereas a coach has to take a long time to manage the players to become more prepared and stronger to face strong opponents in the UCL but unfortunately PSG management doesn't have this kind of thinking so it's a waste of appointing Zidane as coach if he ends up being fired without being given a second or third chance.
previously, PSG had made an offer to Zidane, but the coach rejected the offer because he realized that building a very strong team takes a long time, not just one season, and if he failed, he would definitely be fired and his reputation would suffer because he was deemed to have failed to make a contribution to PSG.