Kylian Mbappe's contract with Paris Saint Germaine makes doesn't allow PSG to extend his contract without his permission and that's the main problem at the club right now because Mbappe has refused to extend his contract.
It now left for PSG management to keep him at the club till when his contract expires by 2024 summer and watch him leave the French capital for free or sell him this summer and make money from his sale. It's actually a big decision to make at the moment for the club's management considering the amount of key players that's left the club this summer
From an economic point of view, this is the simplest solution - the sale is profitable and this is the only option in the circumstances. Another thing is that Mbappe is a huge brand and losing it so unexpectedly is very difficult.
Well, everything else is simple for PSG: they collapsed in the last two seasons, the “stars” fled and they can only admit that now they must build the team again. In fact, it looks quite optimistic - the stellar project has failed and it's time to build a normal team. Galkticos have yet to succeed anywhere.