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The financial situation of PSG is what makes them different from other clubs of Ligue 1. The difference is honestly too much, even more than Bundesliga. Bayern has dominated the Bundesliga for a long period of time, but the competition gap has not been this much. I don't think Mbappe's leave would stop them from winning the league, but it would definitely be tougher to win it.
However, PSG has already said that they would sign 3 new promising players. If Enrique does his thing and creates team cohesion with his new team, I don't think there would be a problem for winning the league. But winning the league has become so common for them that it does not feel like an achievement anymore. They have struggled to win the Champions League even with their prime team so I think the focus should always be on that for a team like PSG.
Maintaining the dominance of the Ligue 1 title is an achievement to be proud of for PSG, even though for some people it may feel ordinary. This competition costs a lot of money and if you don't achieve success at the end of the season, of course you will lose financially. The league is basically more than something that is taken for granted, but of course the Champions League title is something bigger because it is an elite achievement between European clubs.
PSG fights every season to win the Champions League title, but their journey has yet to produce results. Maybe at some point PSG will win the title, but this season they are still not one of the favorites expected to win it. PSG must beat Barcelona in the quarter-finals, if they are able to then they are one step closer to the final.