I don't think people defending this decision by saying "he'll be 26 in 2025, he can just move to a historically major club then" are being very realistic about how big transfers work.
For any given summer transfer window, a top player will have one, two, maybe three clubs they could move to. Those are the clubs that can afford to sign them and want/need to sign them in that specific moment in time. If we consider whether the player might even want to join that club or not, it goes down to one or maybe two. Hence why you rarely see bidding wars or big competitions for these players, it is almost always just the one club that's trying to get them.
For a normal player with a five-year contract that can leave with a transfer fee, these clubs can change every summer depending on the circumstances, so they have some options every year. For a player wanting to leave on a free transfer, there is only the one transfer window, so the options are much more limited.
Even if he took a paycut and accepted his current salary, there are few clubs in the world who could afford Mbappe in 2025. Who is it, really, other than Real Madrid (who I strongly doubt are going to do it)? Manchester City? Manchester United? Bayern Munich, maybe? Barcelona, if they've fixed their financial problems? Liverpool, maybe? Juventus, maybe? Chelsea, if their new ownership is up for it? That's two YES and five MAYBES. Of those teams, two are most likely to be top projects in any given moment (City and Bayern) and so they'll probably be no-brainers in 2025. But the rest go up and down all the time.
And then sure, Mbappe can be very good. Maybe he'll be even better in the next three years. But quality only goes so far. Teams will not sign you and pay a bunch of money unless they think they need you, even if you're great. So you are dependent on circumstance: which of these teams is going to need me, and will have the money to buy me, in that one single transfer window.
