But, I do believe you're more or less making my point for me. All these truly fearsome monsters are fearsome because they bypass the combat rules - all save or die, or just die.
I think that's what I was trying to say before - in combat, after about 4th level, it wasn't all that common to whack a PC. By stuff that did an end run around the combat rules? Oooh baby. Lots of dead PC's.

OTOH, I look at 3e, where monsters, as a rule of thumb, do about 10xCR damage as a max per round. Sure, they aren't going to do it often - that is max damage after all. But, the problem isn't that they have to do it often, it's that if they do it once, a PC dies.