A lot of people pooh-pooh this, saying "Mahomes didn't need coaching -- he was immediately a HOF player out the box!" I think that take is misguided.
It's not that Kafka had to take Mahomes from a lousy player to a great one. What Kafka had a big hand in (yes, alongside others like Andy Reid and Alex Smith) was maximizing Mahomes' natural gifts and teaching him to prepare like a pro.
That's what a team should be looking for in part -- a coach that can maximize what they have. If they have Mahomes? Awesome, they're part of a Super Bowl squad. If they have Daniel Jones? You build him up having career highs in several passing and rushing categories, put up a winning record, and win a playoff game. You have Tommy DeVito and Tyrod Taylor? You wring out .500 ball with effective game management and about league-average passing efficiency.