Not sure what you're on exactly. It feels like you're trying to oppose my points, but honestly I'm not sure.That's the thing, whatever plot armor mechanism you settle on, it'll either make only that last bullet a 'real threat' - or it'll fail as plot armor (at least some of the time, a "protagonist who shouldn't die at this point in the story," will).
Meh. RQ, for instance, did scads of things differently from D&D, not just eschewing a plot armor mechanic. Armor absorbing damage, skill-based instead of class/level, completely different take on magic, etc, etc...
The 'problem' (emulating genre with a creaky old RPG mechanic) isn't insoluble, it's just not soluble by making firearms into instant-death wands, or treating hps as structural integrity units instead of plot armor (or something that can be a fig-leaf for plot armor, like 'luck,' which notoriously runs out).