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Ligue 1 will not lose its popularity but PSG will lose its dominance. I actually think that without Mbappe, other teams will have the opportunity to eliminate PSG's dominance. Yes, PSG is still the #1 favorite for now, but at least they won't be as dominant as they were when they were with Mbappe. We might be able to see where PSG has to fight to win the league championship, unlike this season where PSG really dominated Ligue 1 with a goal difference of +48 goals and collected 76 points, 9 points ahead of Monaco in second place.
Of course, Mbappe is an important player. His deficiency will affect Paris badly, but Paris is a team with financial strength. That's why there will be a transfer to PSG to fill his absence. League 1 will still be a good and followable league, but the fact that there will be competition will make the viewers even more enjoyable. We may not see the dominant PSG leadership after Mbappe leaves.
In fact, when Mbappe last season was not in the PSG squad on several occasions, PSG more often got bad results when Mbappe was absent. Therefore, it will certainly still be a difficult season for PSG to get good results next season, because at least PSG will also still adapt after Mbappe departure. Indeed, I also admit that PSG will still be able and easy to bring in their target players in the transfer market later. But still, for me personally quite sure, even though PSG still has a good squad and manager, but to be consistent and to dominate Ligue 1 without Mbappe it will still be difficult. Therefore, this factor will open up a good chance for other teams to be able to enter the team in the title candidate next season.