PSG without Mbappe will not be as strong as they used to be, and winning the League 1 title is possible but might be a bit tricky and difficult because we have seen teams that tried to compete hard against PSG, and if these teams keep the pressure next season and PSG decided not to sign more players before the beginning of the season, then Brest, Lille, and some teams might win the League ahead of PSG.
I am very sure that PSG will use their money to sign more players to get a good replacement for Mbappe and some world class players that will keep the team going so that they will maintain their reputation of been the best team in the league.
I don't sure PSG will get downtrend performance for next season after leaving by Kylian Mbappe, its difficult decision accepted by PSG after Mbappe not extend his contract but Luis Henrique will make PSG keep dominance in Ligue 1 and reach higher level round in Champion League next season. Have break out almost two months before next season begin I think PSG start to sign top players for replacing Mbappe position although they can put Goncalo Ramos as central forward for Mbappe position.
All top teams when leaving by their favorite players can recovery well and the empty position replacing well by other player, I remember with Real Madrid after leaving by Cristiano Ronaldo replace well with consistent perform of Karim Benzema. Not close possibility with PSG performance for next season better than their performance in this season although leaving by Kylian Mbappe.