IMO:
-You have to be mentally and physically gifted to be a top QB in the NFL.
---mentally and physically means a lot of things, and its a rare beast that checks all of the boxes.
-You have to be surrounded by a team: organization, coaches, players.
-The team must play to the QBs strengths.
The problem with "the next great QB" is that they usually go to a crap team: crap owner, crap GM, crap coach, crap roster... They spend their rookie year running for their lives, throwing to slow, brick handed WRs, and handing it to an old, never has been RB. They really aren't given much chance to be good, let alone live up to their potential.