It is true that if Mbappe leaves PSG on the days when PSG is pursuing every competition that is followed, it will be very difficult for PSG, because in my personal opinion PSG also does not have a replacement for Mbappe to perform well at this PSG club. But this is still just a rumor, we can't be sure what the future will be like after all these rumors happen, whether mbappe will continue to stay at PSG or indeed leave in the January transfer market later, and basically also that real Madrid does need mbappe to strengthen its squad if you think about it, but we are really confused by mbappe's behavior which always gives a controversial attitude.
Now Mbappe will definitely not be able to leave PSG because he has a contract until the end of the season, but he can already start negotiating with other clubs. I read the latest news today that Liverpool wants to get Mbappe, I don’t know if this is true (as the French publication Le Parisien claims), but it seems that Real is still the number one contender. I don't think PSG will try to stop him, they will get big money for Mbappe and it looks like they are already looking for his replacement.
maybe Mbappe stay at PSG can help his team win the UCL title (even though it's impossible) but the fact is that PSG will still have difficulty if they only rely on Mbappe because they are not strong enough if they only rely on one player without having a solid line up. meanwhile, to be able to win a title at UCL you have to have strong cooperation, whereas PSG doesn't have solid cooperation for me.
regarding Mbappe move in january, he won't be moving, but if PSG keeps him until June, it looks like he will leave as a free agent and PSG will suffer a loss if he leaves as a free agent.
It will be hard for Paris Saint-German to win the Uefa Champions League trophy. Paris Saint-German has built its team around Mbappe, and if Mbappe is not in form Paris Saint-German will not perform well.