I've said this but it bears repeating. When Tom Brady was making peanuts, he was an excellent QB but didn't put up otherworldly numbers. As he started making bank, the increase in salary came with a gigantic increase in production.
2000 - Brady signs a 3-year contract for $864,000, plus a $38,400 bonus. But it only goes from 2000-2001 because....
2002 - Brady signs a new deal. 5 years, $30.5 million, plus a $10m bonus. But it only goes from 2002-2005 because...
2006 - Brady signs a new deal. 6 years, $60 million, plus a $26.5m bonus. But it only goes from 2006-2009 because...
2010 - Brady signs a new deal. 4 years, $72 million. But then in 2013...
2013 - Brady signs an extension. 3 years, $27 million. Then in 2016...
2016 - Brady signs another extension. 2 years, $41 million, plus a $28m bonus. And then finally...
2019 - Brady signs a new deal. 1 year, $23 million.
So he was super cheap from 2000-2001. He was definitely more expensive from 2002-2005, but not crazy so. That went up considerably in 2006, and from then on was making big money, sometimes making the most in the league (that 2010 contract was the largest in league history for a brief amount of time).
But here's the point: during his early, cheap years, his production was good, but not amazing. From 2002-2006 his average season was:
61.6%, 3,743 yds, 25 td, 13 int, 88.7 rating - and his team had a .738 win percentage
Obviously he won a ton, but his individual stats weren't THAT incredible. Then he exploded. His salary skyrocketed and so did his production. From 2007-2019, his average season was:
64.7%, 4,417 yds, 33 td, 8 int, 100.8 rating - and his team had a .788 win percentage
Ok, all that to say, now let's look at Mahomes. Mahomes is on his cheap deal right now, which has allowed KC to retain all this talent around him. And his three-year stat line is insane, as he's averaged:
66.1%, 4,622 yds, 38 td, 8 int, 109.3 rating - and his team had a .826 win percentage.
It's very very difficult to picture Mahomes' actual on-field production getting much better than that. And it's very difficult to picture KC's team success being better than that. Especially when, as his salary skyrockets, he's sure to lose talent around him. I mean, honestly, how much better can Mahomes possibly be? Does anyone think he'll throw for 5,500 yards and 55 touchdowns every year? That's crazy talk.