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.
Yes indeed, it would be better to give him more rest time because after all, the fact is that PSG also still has a schedule to play two friendly matches playing against Sturm Graz and also playing against Leipizg. Because anyway, one more match is actually the final match of the France Super Cup which brings together the teams that managed to finish in first and second place at the end of last season's standings. So, the France Super Cup final match will bring together PSG vs Monaco but unfortunately, based on the information available at this moment that the match was postponed, but I don't know the exact reason.
Thus, PSG still has two friendly matches and it is also not bad at all because after all, PSG is also still very visible to have a good team if for competition in Ligue 1. After all, making preparations for the Champions League can basically be done when the new season has started later, so there is nothing wrong about PSG not having a friendly match at all so far.