No player is ever getting less money unless they agree to it, you are implying that the Chiefs are doing this on their own, and Mahomes has no say in the matter. Mahomes and his agents are agreeing on every move the Chiefs are doing to his contract, or the move will not be made.
The Chiefs have already stated that they are going to have to rework the contract down the road, but right now the cap hit is too large to do it. I read it will be around 2025/6 when they will start to rework his contract to ensure that he is the highest paid player in the game.
I think there is a consensus with Mahomes and the Chiefs that they will do what works for everyone. The Chiefs will rework the deal to ensure Mahomes gets paid what he needs while Mahomes takes 'less money' than what he could get from football as he recognizes the power of brand and image.
It's nice when both team and player find a situation where everyone wins and work together mutually. Doesn't happen enough in sports.