Risky move by the Chiefs, imo. Sure Mahomes is worth that contract now but in a sport as physical as football, one never knows how injuries may affect a player in the long run. Also, so much success, so early in his career, can lead to a drop in Mahome's competitiveness and focus going forward - getting to the top is relatively easy, staying there is much, much harder.
A lot would depend on the language but I don't imagine getting out of a contract like this would be easy for quite some time, especially if post-SB emotions played a big role in how that contract is structured/worded.