The worse mistake PSG will be making is letting Kylian Mbappe leave the club for free next year. I am sure they know the consequences of letting him out his contract, they will be losing so much rather than making any profit. It will be highly profitable if they could get Real Madrid to sign him this summer or in January but preferably this summer with them not facing pressure to sell for a lower price.
You're right. It will be a bad decision for PSG if they don't sell Mbappe now and Mbappe refuses his contract extension for PSG next year. Mbappe probably moves to Real Madrid with free status, PSG will get nothing from Mbappe transfer. So, it is better to let Mbappe leaving in the current transfer window since his price is quite high, PSG can get a huge amount of money. PSG can buy some big players from Mbappe's transfer fees, they can rebuild again their squad. However, we don't know what PSG management wants from Mbappe, they may still expect him to stay longer time.
Well Real have 2 more months if they want to buy Mbappe, but so far I understand they can't agree on the price with PSG. Although if you consider that he can leave for free in a year, I guess PSG will have to lower their expectations on the amount of the deal. If they manage to sell Mbappe for 150-200 million euros it would be a huge success.
Sure. It should be a big success if they think about the money. But PSG management may not think about the money, they only want Mbappe to stay whatever the risks. Anyway, I don't think PSG wants to let him leaving with 150 million euros, they must expect 200 million euros at least.
Judging by the news, other clubs have no interest in Mbappe apart from Real Madrid.
Other clubs are aware that Mbappe's price is too expensive. Real Madrid is still interested in him because he is the ideal striker that seems suitable with Real Madrid game style.