Dumb or not, it's those opinions and decisions that determine what will actually occur. If the collective opinion of the 32 teams is that a guy isn't very good, and all of them make the decision to not play the guy, then he's never going to meet your lofty expectations.
And yet you never keep going. You just repeat the same small list of examples as if they are the rule rather than the exceptions.
And if he does nothing more than make a great living holding a clipboard, what then? Does that make him a failure? Or does it make the team(s) foolish? As you've made clear, it wouldn't make you wrong, it would make the team wrong.
Out of curiosity, are you of the belief that an NFL team should use one of their 16 games to play a mid-to-late round pick just for the sake of seeing what happens? You keep saying that guys "haven't had the chance," but what do you consider "the chance?"