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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.
I think ligue 1 is not a league that has quite high popularity in the other 4 European leagues, if we pay attention to ligue 1 maybe we can interpret if they are at the lowest level in terms of the popularity of the current European league league due to the uncompetitive competition so far, ligue 1 is not a competitive league while PSG is the strongest and dominating team so far in ligue 1, Mbappe, who has been a mainstay player of PSG, of course has a huge contribution to PSG, but his departure will certainly not make PSG lose dominance in ligue 1 next season, since PSG became a rich team in ligue 1, of course they already managed to dominate ligue 1 even when Mbappe was not a PSG player at that time.
Ligue 1 so far does not have a team that can indeed jeopardize PSG's position as a dominant team, because indeed apart from PSG, the team team in Ligue 1 does not have the financial strength to be able to compete with PSG so far, even without Mbappe at PSG next season, PSG is still the favorite to win while other teams will only continue to compete for a position to qualify for the Champions League and Europe. But if you say PSG without Mbappe then they will have a hard time in the Champions League, of course agree with you.