How is this a win for PSG? They are not getting to keep one of the best players at that age, dude is a player that can provide you with top notch quality for 10+ seasons and you are losing him and getting Ronaldo that will play maybe 3-4 more seasons which I doubt will even happen, but if he does than there is nothing impossible for Mbappe to provide nearly 16-17 seasons as well, so you are literally losing time here.
Don't you know that Mbappe always rejects PSG's contract extension proposals? If Mbappe persists in doing that until his contract expires and PSG insists on not selling Mbappe then PSG will lose Mbappe for free, which means this is a big loss for PSG.
If at this time PSG accepts an offer from Real Madrid then PSG will get income that can be used to buy other star players. After all, keeping a player who has no heart towards his team is something that is not good.