/snip
Again, I"m not talking about just a saving throw. Did you not even read my OP? It's much more than that. It's knowing that a single breath weapon could take out your whole party, or that if the undead hits you, you lose levels, and then defeating those encounters because you did extra planning and got more creative other than just typical attack rolls. That's what makes it more heroic. Facing tougher odds and coming out on top, as opposed to facing mediocre odds and winning.
Meh, if your 10th level MU had 25 HP, then you were doing something wrong. By that time, you'd found multiple wishes, probably a magic pool or two, and likely something else that granted immunities and whatnot. I mean, a simple Protection from Fire spell (or is that the 2e version, it's been a while) protected you from like 80 points of fire damage. Never minding that gaining magic items that gave immunities wasn't all that rare. Oh, and nice cherry pick of choosing the one dragon that could do that kind of damage. Let's ignore things like 5th level parties killing ancient huge black dragons (DL 1 - ends with an ancient huge black dragon).
But, again, nice cherry pick. Let's pick the character with the LEAST hp to show how dangerous the game was.

That 10th level fighter probably ran somewhere towards a 100 HP (he'd have at least a 16 Con by that time and probably a 19 or a 20 given the PLETHORA of magic items). I'd point out that actually, there's zero difference in HP between an AD&D 9th level fighter and a 5e one.
OTOH, the monsters dealt about 1/4 of the damage going from AD&D to 5e. Maybe 1/2. See, you keep saying it's not about the save or die effects, but, ignore the fact that 1e monsters were puny compared to their 5e counterparts. Less HP, less damage, hit less often, etc. Compared to their 3e counterparts, AD&D monsters were a bad joke.
The only reason the odds were tougher is because the game was designed to bypass the combat system in order to make creatures threatening.
Tell you what, run a spectre against a 7th level AD&D party, but, take away it's level drain. It's not scary. So, the only thing scary about it was the fact that the designers had to basically create entire subsystems to bypass character power in order to be an actual threat.
Every year we see this same tired old tripe pedaled out again and again. Things were so much harder back in the day. We had to walk uphill in the snow both ways just to reach the stick that we'd beat ourselves over the head with. It wasn't true then and it certainly isn't true now.