If Mbappe doesn't extend his contract with PSG then keeping Mbappe in the squad for only one season will not be profitable for PSG. My opinion is that coach Luis Enrique should be given a chance to work with a completely new squad. If PSG make Mbappe part of their match plan this season, and if Mbappe leaves the team next season, then PSG's performance will be erratic again next season. So Luis Enrique should work with the new team this season. Not keeping Mbappe in the match plan. Maybe PSG's performance will be erratic in some matches this year. But if the team gradually stabilizes, they can continue to perform well in the coming season.
Mbappe's relationship news seems to me to be just rumours. We have seen a lot of drama regarding Mbappe. But Real Madrid need an experienced striker if they want to dominate the Champions League.
On the one hand I think you are right because building a squad with Mbappe being at the core of their whole game has its downsides when Mbappe will leave next year anyway. But isn't it often the same when great players are part of a team? I think they should rather try their best with Mbappe and accept that he will be gone after this season. As long as he is still a part of the team, it wouldn't make sense to play as if he wasn't there. If Mbappe plays his best for the team and pushes his team mates, everyone can benefit from him as well and I think that is what it's all about when you have superstars in a team. How should they even do it? Let him sit on the bench? That would be even worse in my opinion and only detrimental to their success. He now scored two goals and that is what he is there for.