To be fair PSG had a lot of players at Euro, which means that those players are already quite fit and not need that many friendlies, and they will have two or three friendlies if I am not wrong, which is more than enough for them since they are going to play a lot of games.
When a team plays like nearly sixty games all season long, adding friendlies to that doesn't make sense, it would be just added fatigue for the players, most of whom ended up playing all through last year and during the summer as well, so it is not going to be easy at all, would be quite tough for them to keep going. So resting them, sending them on vacation and tell them to relax, would be a lot better.
That way, you are going to get fit players anyway, not like the yare going to suddenly gain 10kg or something, these are professionals and they played 60+ games last year, so this way you are at least giving them some time to be happy and relax and find some peace. Then, you could tell them to come back, have 2 friendlies, and then start playing at the league. It would be a shock if they lose their early games, but even if they do, let's face the reality that they will end up winning the league easily, even without Mbappe.
I think it's quite obvious that we are going to see PSG have a little bit of struggle at the start of the season, not just because they didn't do many friendlies which will definitely be one of the reasons, but because they are forgetting that they do not have Mbappe anymore this season. Point of playing friendlies this season wouldn't be just to get back in shape, it wouldn't be to keep players happy or whatever, it would be because for the first time in many years, maybe first time for any of this PSG players, there won't be any Mbappe on the team. So they should have played some friendlies to see how they would fair without Mbappe, if they could do that then it would have been a great deal and I think it would have worked out very well.
I think the best thing to do right now would be just focusing on how PSG could work things out without him, and that requires a lot of training and players realizing what that means. They all will play differently, and they won't be just giving the ball to him, meaning they will play a lot more team ball, and that requires a lot more training as well. I think it should be noted that we are going to have some trouble without these friendlies for sure.