The salary cap increases every year, Mahomes will be 36 when the new stadium opens, and the team went from Dawson to Mahomes struggling at the position except for a few years with Montana and Smith. They do not have a young QB that they are developing and shouldn't when you have a 30 year old QB.
You rework the deal, expend him a couple more years, and when he is 38 you start worrying about the future.
More than anything KC needs to seriously look at the offense, and reshape it in a way that gives Mahomes more protection from taking hits next season, they have a young offensive line that should be fine for the next 3 or 4 years, they need to find a replacement for Kelsey, maybe one more WR but more than anything they need a quality RB that can take the load away from Mahomes. Reid then needs to stay committed to having that back run the ball or catch it out of the backfield, 20 times a game. This relying on Mahomes to make a play and save the drive has to become less and less.