Although they don't have the world's top players like they used to, the other clubs in the French Ligue 1 are much worse. Their players are mostly not comparable to PSG's players, although there may be some who are better, but most are not. PSG's front line, although they don't have Mbappe or Messi anymore, they still have Dembele and also Kvaratskhelia.
The quality of the players in other teams is below PSG players so even though Lionel Messi and Mbappe have left, it does not weaken PSG's strength and in fact they are getting stronger because the front line is much more productive. Dembele is in top form this season and he looks so productive in scoring goals, this is quite an impressive season for him.
If there is no competitive competition, PSG will always dominate Ligue 1 and this is not something difficult because there are no challengers from other teams who are able to beat PSG's dominance throughout the season.