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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 don't agree with you, Ligue 1 will lose popularity if there are no world class players playing there, the LFP as the League authority is still having difficulty finding parties who will broadcast matches there. Kylian Mbappe departure will have a big impact on the popularity of Ligue 1, LFP really hopes that PSG can bring in several quality players to increase the sales value of the competition.
PSG will not lose its dominance in Ligue 1 even without Mbappe, Mbappe departure will not leave a big hole in the PSG squad because they still have other quality players. PSG can bring in other players with the money they have, they are able to make Mega transfers without any financial problems. It is very unlikely that PSG will lose their dominance because they still have time to bring in other top players before the League starts.