Mahomes still has a chance to catch Brady, but there are a lot of factors that make it fairly unlikely:
1) Longevity: Mahomes' playing style won't age as well as Brady's, clearly. He also doesn't seem to take care of his body the same way. Beyond all of that, Brady was also (a) lucky, and (b) a freak of nature. Other pocket passers clearly degraded in physical abilities over time - see Manning, Brees, Rodgers, Stafford, Rivers, Ryan, etc. The top 3 guys had some devastating injuries that caused that degradation, but the other 3 didn't and clearly still fell off. Brady in his last 2-3 years essentially looked like the same guy with the same pocket mobility and arm strength, with maybe his last year arguably showing some drop-off but that seemed to be more related to his divorce (he seemed to have lost a lot of weight during his "retirement/unretirement" offseason). So even if Mahomes makes the transition to pure pocket passer, it's no guarantee he will last nearly as long or as well as Brady did.
2) Coaching: Reid and Spags are in their mid-late 60s, with Reid being 67. That's equivalent to Bill in 2020. Will Reid coach another 10 years? Very unlikely - probably 3-5 tops. So Mahomes will have to weather a major coaching change that Brady never had to (until he left for TB).
3) Roster overhaul: the early Chiefs were powered by a core of superstars (Hill, Kelce, Jones). Hill is gone, Kelce is basically washed, and Jones is 31 at DT, meaning he's got maybe 1-2 years max of prime left. Similarly to the Patriots, the Chiefs are going to have to rebuild the roster not just from the starter/fringe level (which happens to some degree every year) but replace their non-QB superstar core. The Patriots faced this from 2008-2010 as all of the old defensive stars aged out, Moss aged/complained his way out, O-Line got completely overhauled. They were lucky to land a Superstar in Gronk and consistent PB-level contributors in McCourty, Hightower, Edelman, etc. That's what rejuvenated the franchise and led to the 2011-2018 period, the end of which came because those guys got old and the drafts from 2013-2018 were relatively barren. Can the Chiefs do it? Of course - the Patriots did. But the Patriots were the exception, not the norm - historically it's basically impossible to recreate your dynastic core, esp. with years of low draft picks and your QB taking up a big chunk of the salary cap.
As I stack it up, I think the Chiefs can squeeze 1-2 years more of contention out of this core but they are absolutely entering the ~05-09 period that the Patriots faced. They also have the additional challenges of the head coach being much older and their QB playing a style that is almost certainly going to age faster. Mahomes' playing style probably won't catch up to him in the next 2-3 years but 5+ is a crapshoot. Rodgers is probably the closest comp and he really hit a lull in the mid-2010s (coincident with several seasons cut short by injuries) until a new HC with a new offense gave him a 2-3 year jump start from 2019-2021 before age & injury finally caught up to him in 2022-2024. I don't think Mahomes at age 35 is going to be bad/Russel Wilson-fringe starter level, he will still be a top QB, but he's going to be worse than he is now and therefore will need even more help than he's had (which is a lot!) to see multiple SB-winning years, which places even more stress on the franchise to add new superstars.
