Kvaratskhelia could be an option for Mbappe's replacement when he decides to leave this season but for Rodrygo it seems a little difficult.
He is still the center of attention in Madrid even if there will be Mbappe in Madrid but it will not interfere with Rodrygo in terms of position so there will indeed be no problem for this player and will remain in Madrid.
There are some players like Dembele or Asensio who are actually still very good but Enrique still can't bring out their potential especially for Asensio so even if Mbappe eventually leaves if there is Kvaratskhelia in the squad it won't be too bad.
For Ligue 1 actually they will not worry too much because after all Ligue 1 always leaves the first place for PSG but to compete in the Champions League it is at a slightly different level so they have to work extra if they really want to be the best because some big clubs are still quite balanced even better than PSG also want the same results in the Champions League.
I do agree that it's quite a good replacement, not sure if the fit would be perfect because sometimes even if a player is great, they do not fit well into a team and have trouble, but if the fit is good then I do not think that they would get worse one bit at all. That is not to mean that he is as good as mbappe, it just means that mbappe had a limit with PSG, he will do better with Real Madrid I am sure, but that limit meant that he is not going to be able to do better than what PSG gives him.
Whereas we are going to see Kvaratskhelia end up being about as equally close to that limit as well, maybe a little less goals, but at the end of the day he would still be there. Plus they would spread the goals around, meaning PSG as a whole will score that many anyway, but with multiple ones instead of just one player.