Mbappe seeing his contract out and going to Madrid for free suits everyone.
PSG get the most out of the messi neymar mbappe trio, both on the pitch and marketing. Getting money for him now is not worth much more to them than keeping him, as they'd have to pay monaco a significant chunk if they sold, and probably would get less than they'd want. Which they don't need anyway.
He gets to play with messi for a year, and move to madrid with a signing on bonus to replace benzema.
Both get a damn good shot at the CL.
And Madrid get their player for fuck all next year. They will do fine with what they have this year, with Barcelona fucked anyway.