Indeed, PSG will only be able to win the Ligue 1 title if they cannot overhaul their existing line-up for next season. In fact, this incident has happened many times and it's still difficult for PSG to win the highest European competition title. Maybe if Neymar and Messi leave PSG it will be something that will definitely have a very detrimental effect on PSG because now the star player is only Kylian Mbappe.
With the current squad, of course PSG will have no trouble winning back Ligue 1 next season, the problem is they will still be eliminated if they still use the same squad for the champions league. PSG management must immediately go out to find several candidates for new coaches and they also have to compete with several other top European teams to get talented players who are currently the targets of several of these clubs.
That's why, in the transfer window last winter. PSG management did not bring in newcomers. however, in the summer transfer window, there will be many players on their selling list, including some of their stars leaving. after that, they "PSG management" will target some newcomers, and let go of some players especially those who don't have much contribution. however, it is highly likely that Mbappe will stay with PSG until his contract expires.
I suspect that apart from looking for a candidate to replace Galtier, PSG will also replace their technical director. after that, they will bring in a number of new players, be it midfielders, defenders, and some other sharp strikers, such as Osimhen, for example, who was linked with PSG. what is certain, PSG has adequate finances to bring in the players they need and want, including even expensive players. that is, the departure of Messi and Neymar will not stop the search for potential replacements. well, let's see what PSG will do in the transfer window next summer.