It is great news for PSG fans. I believe Nuno Mendes is currently one of the most valuable players for the team. He is only 22 years old but performing in a spectacular way for his role.
This must be the priority for PSG from now on. They shouldn't lose their crucial players easily. If they maintain the quality level and continue to add the right players, they can finally achieve their ultimate goal one day. The goal is a Champions League trophy of course. Yeah they couldn't win one even while they had a dream trio like Neymar - Mbappe - Messi. However this doesn't mean they won't with different players either. The squad is very young so it just needs to be given some time to go for bigger achievements.
I think the time is right for this transfer and while many people did't see this coming, it was the right decision for him.
When you say that they shouldn't lose their crucial players, at the end of the day it will be equalized when they get another player for him. Money isn't a big deal for them and they will purchase a player as a substitute no matter what the price is for them.