Mahomes needs work. His target date as a starter might be 2018 or even 2019, which eliminates a huge chunk of the surplus value he offers as a rookie quarterback, given that he could spend a couple of years on the bench behind Smith. He's about as high-risk of a quarterback as anyone has taken in this draft in years, although the Texas Tech product also obviously offers enormous upside. Drafting him incurs the opportunity cost of not upgrading with a wide receiver or a cornerback for a team that could be very close to competing for a Super Bowl right now.
http://www.espn.com/nfl/story/_/id/19261281/how-lure-low-cost-qb-caused-bears-chiefs-texans-trade-2017-nfl-draft
So the data says don't draft a QB because Kansas City could have got another WR for Alex Smith?
The whole analysis is flawed. The goal is to win the Superbowl. Andy Reid decided to put his chips all in because he understands the goal of the game and any great coach. He decided that Mahomes could be the QB to win the big one and Alex could only go so far.
Now for other teams winning a Superbowl might not be the goal but just winning enough games to keep their job. That's why you want people at the top of your organization who are confident and have the right goal.