The TLDR is a well known axiom: At any time there are only 4-6 "Franchise QB's" active in the NFL.
If you don't have one of them, you don't have a QB.
And everybody wants/needs one, so QB's become overvalued and selected higher than their talent/potential would indicate combined with a rush-to-play-them mentality, leading 80% of top-pick QB's to fail.
This has been the way of the NFL for ages now. Only the rush-to-play ethic has gotten slightly worse.
But such a top-down analysis fails to breach the real reasons most QB's fail, almost always the specific scenarios they find themselves in, on bad teams, who rush them to play, with inferior talent supporting them, and coaching befitting a bottom-tier, high-drafting team. When placed in such a scenario, any weaknesses will be exposed and the player will be a bust if time is not given and additional support not provided.
I.e. TLDR: Most QB's bust because their team and coaches are poor and don't improve in their tenure. Their own failings are a factor of course, but not always primary. And these busts almost always will be shoveled off to backup status or to a second chance on an equally bad team.
What this speaks to is that perhaps building up ones team as a whole should take priority over taking potshots at QB's in the early rounds. That overall team talent may be more important, that coaching and patience may be far more required then not, that adequate positional coaching is a must, and that QB talent in depth (but perhaps not 1st/2nd round depth) is vital. That giving young developing QB's time on the bench to learn before they play needs to return to the NFL.
With all that said, some QB's simply won't succeed in the NFL no matter the support. No amount of excuses or explanation will change that players inherent faults, flaws and inability.
The trick for a GM is knowing where your QB lies, is he a failure due to lack of support, or does his play (even with minimal support) shows flaws that no amount of support can overcome.