-snip-
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.