I mean what does "permanent and irrevocable" actually mean? Why cannot someone, perhaps another PC, bring these characters back via magic? At least in theory it should be possible. Granted, it might be much later, but still.
I've been very clear about what it means. A permanent death is one that isn't going to change "on its own"--by which I mean there won't be any third-party intervention, nor is the death only a temporary setback. Conversely, an irrevocable death is one that the other PCs cannot reverse, meaning they either lack the ability or the resources to raise their dead ally. Gandalf's death was irrevocable (nothing the Fellowship could do about it), but not permanent, because Iluvatar sent him back. Westley's "death" in
The Princess Bride is permanent but not irrevocable, as they were able to petition Miracle Max for his aid, albeit dealing with his hilarious curmudgeonly antics.
As you note, if the party can be resurrected by someone else, then of course the death is revocable
in some sense, but is it actually, practically so? Who will do it? Especially for a party of low-level nobodies,
who is resurrecting them? That's what my efforts as GM are meant for. They bridge that one final gap, answering the how and where such things would come from.
These days, because the PCs have become outright legendary, there's no end to the number of people who might potentially try to resurrect them, even if it was at great cost. But when they were first starting out? When nobody had access to a resurrection spell and they were just random nobodies? I needed tools. So I fashioned them.
I don't expect everyone to do that. I don't even know if a majority of groups would benefit from it. But I do know that a lot of groups
would benefit from it. A little bit of forethought and preparation goes a long, long way--and it saves DMs from having to use far more disruptive tools like railroading, fudging, "quantum" monsters/locations, or outright "retcons" of things that happened in-character.
As for when a death would in fact be both permanent and irrevocable? Consider:
- PC dies at level 1-4. Revivify isn't available to anyone yet, and absent the work I just spoke of, the PCs have no other recourses. Doubly so if it's a near or full TPK.
- Level 5+, PC dies, body lost/destroyed/undead-ified. Only true resurrection can save them. Only a fantabulously wealthy party could afford even one such spell...and they're dependent on finding a 17th+ level cleric.
- At any level, the PC who died is the one person in the party who could cast resurrection spells of some kind. Kinda hard to raise yourself, y'know?
In all three cases, only two factors can come in to fix either permanence or irrevocability: a "friendly" (note quotes) NPC's intervention, or a mysterious power. The former creates story by having the party beholden to the NPC(s) in question that brought the character(s) back to life. The latter creates story by leaving a puzzle for the PCs to solve,
who or
what saved them? Can it be replicated? Can it be
controlled? Etc.
It doesn't require a lot of GM effort to plant these seeds. E.g. make a big cathedral to a friendly deity, say its archbishop is widely known for being a powerful cleric, and have the party do at least one quest on behalf of that deity's church--boom, instant 17th+ Cleric who may now have a reason to help. But it does require
some effort. It is not something that just spontaneously manifests out of the blue.
And yes, you have correctly noted that, especially at higher levels in WotC-era D&D, this process requires very minimal effort! Now I hope you can see why I would get so cross when people openly insult me with questions like "OH SO NOW EVERYTHING IS JUST AN AUTO-WIN FOREVER HUH?!?!?!"