I was raised on bad BYU defense, which was OK because, like you say, we had QBs who could win in a shootout. Despite TH's freakish running ability, he's not that QB to pass us back from big deficits.
Under Bronco, the offense will never be stellar, but defense has been solid in his tenure. When you see him give up the reins to a less talented buddy, it's beyond discouraging. Move after move are just plain amateurish, like the Bronson experiment -- any HC worth a yewt would have moved him back after analyzing film from a couple of games.
It's not just this season of excruciating failure that has people fed up, it's a culmination of factors over years of mediocrity (inability to ever win the yewt game, lack of player development, etc.) shrouded by the appearance of successful seasons.
So, no, it's not just about an unfortunate string of injuries this season.