The Mahomes deal is a lot of money, so why pay him so much?
To begin with, KC secures a top rated QB with a ten year contract extension. That is 12 years with this QB before they have to make another deal. At that time he may be the lowest paid franchise QB. Also, he will generate 10 times his contract with TV and merchandise sales.
Next, Mahomes may not receive the full $503 million. Over $100 million is in incentives to be paid if and only if he meets the incentives stated in his contract. Things like League MVP and TDs in a season, yards thrown, spring workouts, and more.
This notable sports contract does break new ground, but so did others in the past in football and in Baseball and in Basketball and in World Soccer. If it's not coming out of your pocket, why complain.