Mbappe is reportedly the highest paid player at Madrid. He wasn't going to Madrid - and isn't at Madrid now - as a "volunteer". PSG likely paid more, but it was eye watering money at PSG as it would have been at Madrid.
The notion that going to Madrid would have "maximised" his sporting potential is mostly anchored by wishful-thinking, yet people speak of it as if it was a certainty. PSG has been chasing a CL for a while, and in 2021 or 2022 they had a team that should have delivered it. He was at home, with some of the best players to have ever played the game, with a decent shout of delivering for his "local" - why the feck not? I'm not saying the money wasn't enticing nor primary motivation, but people always post about the matter so definitively on some "Pep or Money" Guillem Ballague style.
Either way, more players need to say no to supposed big clubs more often and instead of joining stacked institutions, try to compete against them.