Real Madrid and Barcelona will continue to compete until the end of the season and in the transfer season there will also be many changes for one of the top teams. If Real Madrid can bring in Mbappe, I am pretty sure Madrid will firmly dominate La Liga.
There was a time when Real Madrid had C. Ronaldo, Ozil, Di Maria, Ramos, Pepe, Benzema, Xabi Alonso, Casillas, and Marcelo were all at their peak and they did not dominate La Liga but somehow you believe with Mbappe they would dominate La Liga?
Mbappe is a great player. Some might argue he's the best winger in Europe right now and I might agree but saying Madrid would dominate La Liga just because of Mbappe is a different thing.
PSG have not convincingly dominated Ligue I with Mbappe, talk more of dominating La Liga with Madrid when Barcelona isn't sleeping.
I would love to see Mbappe in the La Liga though because it's obvious his career is not heading anywhere in the French league.
At that same time Barcelona had only Messi as the game changer and move maker for them and with him alone the dominated the la Liga so don't doubt it's very possible that if Madrid get mbappe added to their squad their dominance in the la Liga will be sure, if you take a good look at the Madrid front line the have issues with getting a good striker and this is a position mbappe can comfortably fill and perform optimally giving good results.
PSG has not dominated the french league this season but you can attest to the fact that the had a fantastic season last season of which mbappe was a key player then, currently you can undermine the strength of PSG in the french league as they are still standing tall regardless nof the recent issues that have been going on in the team but them mbappe still be doing his best.
I would even love to see mbappe in the la Liga as I feel he will explore his talent better in la Liga, because the french league is almost reducing his value as that league is gradually turning into a league with so much mediocrity if he leaves I think it will be even better for him as he has stayed there for just too long, I remembered there was a time when Messi had to leave Barcelona there were speculations he wouldn't do well outside Barcelona but am sure today they could see for themselves how well he is doing.