I mean, just look at Montana's tenure with SF. The guy who, before Brady, was the GOAT.
He played from 1979-1992 in SF. That's 14 years. One was his rookie year where he hardly played, and he missed basically two other seasons due to injury. Which, of course, happened to Brady as well (1 year), and which could happen too Mahomes. So in the other 11 seasons, SF:
- Won the division 8 times.
- Went to the playoffs 9 times.
- Won the Super Bowl 4 times.
And that was (1) in an era when they could really keep teams together, and (2) the greatest run in NFL history over such a long period of time, until Brady came along.
If Mahomes ended up having the same kind of success Montana did, that would be remarkable. And if he did, he'd still fall FAR short of Brady's accomplishments.