Hey Tarka, and welcome the forum.

I'm glad you're enjoying Demon.

Demon actually use to use that rule set, long long ago. It was removed because it was forcing the balance of the game to be a little too tight: the changes I was having to make to ensure a death from full HP without a chance to respond could *never* happen were slowing the pace of combat too much. Once I added the MaxHP=>prevent Death mechanic, I was able to increase overall damage a fair bit, knowing that the mechanic would provide a cushion for what would otherwise have been deaths from full HP without a chance to respond.
If I turned (or gave an option to turn) hardcore mode on, without undoing the balance loosening I was able to do after removing it, there would definitely be cases where, in hardcore mode, you would be able to die from full HP without having a chance to respond, if you were playing low defense builds or happened to run into an extra nasty Modified demon or demon pack.
I admit I'm divided on whether or not that'd be okay. I feel like if I offer a game mode, that game mode needs to be balanced and fair, and without making a lot of other changes, putting the HP = 0 = death rule back wouldn't be.
While we're talking, is there a particular reason you'd prefer this mode of play? If it's just personal preference that's fine too, but if there's more to it than that I'd love to hear about it.

Thank you for the suggestion!