2 big differences. Brady was in his age 36 season in 2014. And a pocket QB.
Josh Allen is 30 and a running QB, the kind that runs QB powers, etc.
Turns out, Brady wasn't done, but it wasn't completely crazy to think it at age 36.
To think there's any sort of window for a Running QB post-age 30, when the Bills are currently over the cap, haven't had a decent draft in 3 years, and have won zero road playoff games in this run, well...you have a lot more faith in Josh Allen than I do.
And Lamar Jackson was the best QB on the planet last year, Allen just won the MVP award. And Lamar Jackson is almost 29.
People just assuming Mahomes/Allen/Jackson would be playing into their 40s at an MVP level were mindbroken by Tom Brady. Name the running QB that was better in his 30s than in his 20s. It's a really short list. It's Steve Young, that's the list. Rodgers and Roethlisberger ran early in their careers, but transitioned to pocket passers by their mid-late 20s.
Mahomes, Allen and Jackson have not done that, although of those three, Jackson is at least trying to. The other two have not modified their games in the slightest, and it will severely limit their primes.