What makes you think that PSG will no longer dominate Ligue 1 now that Kylian Mbappe is gone?
I don't think Mbappe is the only player who can score goals when he is the target man. Other players such as Goncalo Ramos and Randal Kolo Muani can also be expected to be good if Luis Enrique makes them target men. So I don't think PSG will lose much to be proud of next season, that includes their goal productivity.
As a result of Mbappe leaving, the two existing center forwards are likely to get main places in Luis Enrique's squad. Ramos or Muani will shine even more next season, so I don't think PSG will lose ground in dominating Ligue 1.
I don't think losing Mbappe will make it difficult for PSG next season, PSG will continue to dominate Ligue 1 and I don't think PSG will have any problems with Mbappe's departure next season. PSG must immediately forget about Mbappe because they have to look for another replacement next season and if there are no players they can successfully recruit then Ramos or Muani will be options that Luis Enrique can use.
Every team will definitely experience the same problem where players will come and go, so this is not a big problem. The problem is how next season they can be stronger in the Champions League competition because the big target that Luis Enrique must achieve is to give the team the trophy.