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.
Your statement is perfect about Mbappé and PSG with most chances we will have his exit from here in June and then PSG will look for the other options and currently their main target is better results in Champions League even with just one player it's not possible to win, but they need to establish a team which give them better approach for this and it's never been easy because it's time-consuming work and management needs to give time for this if they fire coaches in every one or two years then surely they will be not able to win this.
Here I also agreed about this most chances he will leave as free agent and this could be big lost for the PSG and good gain for the club which is going to be able having contract with him with nothing is sure about this because I have feeling now Real Madrid fans will be not happy with him after his two U-turns in the past.