Injuries to key players could very well result in the team being inconsistent in winning matches so that teams like Nice and Monaco are often easily left behind by PSG, who are still kings in Ligue1. But if this is caused by player fatigue, I think we need to look at the playing schedule of the two teams this season in whatever league they are in. Because if both of them only play in Ligue1, I don't think their players will get tired easily because other teams' players do too, including teams that have high consistency in Ligue1.
Maybe what you say is true, that injuries to key players will result in teams like Nice and Monaco being inconsistent and therefore unable to compete with PSG. But in my opinion this is not a matter of player injuries, they are unable to compete but because PSG is too strong for them, especially in their squad of players.
PSG is the biggest team in the world, although currently PSG cannot match the records of Barcelona, Real Madrid, Munich, Man United, Liverpool and Juventus. but because they have a lot of money they can buy players easily, and in the end the French League is less attractive because no one can compete with PSG. So it is not surprising that if the domestic league in France increasingly dominates, the hatred from supporters of other clubs will become greater because other clubs will no longer have the opportunity to compete for the league title.