If Mahomes is rational, and has rational advisors, then I think it would hinge on how much endorsement money he's making now, and how much he can make in the future, and how much (if at all) that outweighs what he can make from the Chiefs.
Forbes seems to think Mahomes makes $20M/year in endorsements, but that was before he won his second SB:
https://www.forbes.com/athletes/