I am sure that they can bench him if they want to but they need the go ahead from the management as well. I mean it's all about how you could end up with getting rid of a player, if you do not play him and make it very clear to him that you are not going to play him, then he will leave himself. This requires the team to realize that they want to get rid of him no matter what.
Mbappe knows this is a threat that PSG cannot uphold so he's not shaken by that threat. There's is no way PSG will bench Mbappe for a while season. With Messi and Ronaldo out of Europe, Mbappe is arguably the best attacker on the continent, so I don't see how a club can have that kind of player and put him on the bench for a whole season. That is a threat that hold no water.
The best that can happen for PSG right now is if they can convince Mbappe to take the Al Hillal deal. That deal solves a lot of issues for PSG. They would not have to pay Mbappe the agreed 65m, they also receive a huge sum for Mbappe and have money to spend on other signings.
I'm just trying to imagine PSG benching Mbappe even the champions league and I don't see a scenario where that happens. It would be a disaster for bother Mbappe and PSG.