If PSG decide not to sell Mbappe, they make a bad decision, because they will miss the opportunity to get income from the sale of Mbappe.
I know PSG is a rich team with healthy finances, but they also definitely don't want to suffer losses by releasing Mbappe on a free transfer.
I am optimistic that Mbappe will be sold this year, because Mbappe has refused contract extensions from PSG.
Mbappe likely reaches his limit and he won't be able to grow more in his skills. I agree with you that PSG can get the best selling price. If he won't sign a new contract, PSG will have to sell him at lower price in next season. Even if he signs a new contract, as being said, if his skills won't grow more, his price would stay at the same and might decrease when he is getting older and older.
After all, in my opinion, why is PSG holding back a player whose heart is no longer at PSG?. Even though he plays for PSG, it is possible that Mbappe will not play wholeheartedly.
Mbappe want to win Champions League and Ballon Dor so he won't feel happy at PSG. Even the club is one of the best in the Euro now, he still want to join Real Madrid, the biggest club ever in Champions League history.