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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.
Most people also think that it will be a concern how PSG will be without Mbappe in the course of the season that will begin, because indeed Mbappe became the core player of the attack that PSG had last season besides Mbappe's presence was very influential in the front line with his speed and ability.
But apart from that I quite believe that the popularity might decrease although not significantly but people will pay more attention to where the star plays and that league will be the highlight, after all Mbappe is one of the stars of football and he has a lot of fans so it is possible that some viewers switch to watch the match especially at Real Madrid where he has a career today.