I've seen multiple people use the QB as an example of how teams don't truly go BPA. The only thing I would say to that is QB is the only position where only 1 of those guys plays at a time in the game...so if you have a QB clearly you wouldn't draft one if they were BPA. I guess that proves the point...but theres shades of grey in there to me.
We could probably make it easier if we removed QBs from the equation. They're somewhat like pitchers in baseball only in baseball you need more than one whereas in football as you posted there can be only one. Kind of like the Highlander eh?