Big difference is
1. Brady doesn't use his legs like Pat.
2. Bill remained coaching. Mahomes will have a new coach.
His quest for Brady is over
Aside from the fact that the "Mahomes chasing Brady" is a media narrative aimed at keeping ratings up post Brady... this is ridiculous.
If we're talking team accomplishments (SB rings) this doesn't change much, KC window is closed and getting to 7 would always be hard.
If we're talking individually, he's not far behind Brady in terms of awards like MVP, All Pro...but stuff like career TD would always be tough as Brady played until 45.
This really has 0 to do with Brady, and you can tell someone got an agenda when they start inserting him.