In the next season when Messi and Neymar leave PSG, they will still have the chance to hire a better place and this can be a reason to say the quality of the French league won change.
I don't think PSG will be a weak team next season because of the departure of several star players this season. It will not have a big impact on PSG as they can bring in several other players in the transfer window. I think PSG will still be the strongest team to win the Ligue 1 trophy even without Messi. However, this will become a weakness in the next season if the players brought in by PSG cannot fully contribute.
PSG have not won the UCL trophy for the first time, even though they have been strengthened this season by many star players but they still failed in the ucl. Will they fail next season again only time and player signings will be the answer.