Lemme give a quick bit of fact:
If you're rolling a save or die, that's a generosity on the part of the DM. You're lucky to even get a chance to live, that's how bad you messed up. You should be dead, but maybe luck will give you a bit of leeway.
Examples:
If you're fighting a Medusa straight on, and it looks at you, you effed up and get to die.
If you're charging across an open field and a death god's priest casts Distintegrate on you, you messed up and deserve to die.
If you kick down a door, and poison gas starts to flood the room, you messed up (again!) and should probably die.
If you have no reason to expect a medusa and start to try fighting it straight on then you only know you effed up when other people turn to stone. The first PC to be hit did not eff up - he took the most sensible course of action available and was killed by a DM "Gotcha".
If you're charging across an open field and a death god's priest casts Disintegrate on you, that's par for the course. Anything else leads to either banning the disintegrate spell or the only effective characters being those that can disintegrate dragons - eggshells armed with sledgehammers playing russian roulette. (Or, come to think of it, adding a long casting time to Disintegrate - so if you don't dodge for 30 seconds you die).
If you kick down a door in a dungeon and poison gas starts to flood the room, that's damn poor dungeon design (poison gas + open room -> dispersed gas; and poison gas + dungeon -> gas throughout the dungeon slightly hurting every monster that breathes down there) and the architect should probably die.
Not
one of those situations you mention is a necessary darwin award. Every single one of those situations is very possibly the result of a jackass DM.
Those DMs who are against save or die: If a player jumps into a volcano, do you let him live?
If so, do you let him live if he jumps into the gullet of a dragon?
If so, do you let him live if he gets stomped by a dragon?
If a player jumps into a volcano, that's something he chose. And if there's a reason for it I'd give him a chance of success (and massive damage and debilitating injuries). On the other hand, if someone is
pushed into a volcano I'd definitely give them some sort of chance of clambering out taking a lot of damage each round as they do. It's not instant death on falling so much as almost inevitable death as they start to sink unless they are helped.
If he jumps into the gullet of a dragon, the dragon starts chewing. Crit bite the first round, breath weapon damage every later round at a minimum. Chance of survival: low.
If someone gets stomped by a dragon, that's normal attack and damage rolls. Not one shot of DM fiat.