I think one season is enough to gain experience from Messi and Neymar. Mbappe is already an accomplished player, he has already played with all (against all) star players in the world, now he needs to farm statistics and achievements to go down in history (competitors like Haaland are not asleep) and it will be more convenient to do this in Real than in PSG (which is season as if it was in the wrong league and plays with midgets and not serious rivals).
Messi is going to last years of his career and his time in PSG is counting down. He can play well in up to 3 seasons and after that, seriously I don't know how he will play. He can score some important goals and decide matches but his importance for club will decrease dramatically when he is ageing more.
Mbappe is a very lucky player. He is not only talented but also belong to a right generation to take over leading positions from Messi and Ronaldo. Mbappe and Haaland are players for next generation. We mentioned a lot about Bale, Hazaard, Neymar but they are long in period when Messi and Ronaldo were in their best. So chance for Hazaard, Neymar and Bale to take over those two greatest players in last 20 years is very very small. It's impossible mission for them.
If they had played in period without Messi and Ronaldo, I think Bale, Neymar and Hazaard would have won at least one Ballon dOr'