I don't know why people keep insisting that because we believe that HP relates to meat that we must fully buy into death spirals.
It's pretty simple, really. If hps do equate directly to the severity of physical wounds, then at some point, those wounds would be severe enough to inflict a penalty.
D&D doesn't have wound penalties, so hps can't be as heavily 'meat' as you'd like them to be. Ergo, you must want wound penalties - and systems that have them do tend to fall into death spirals. It's one of the down-sides of such systems. For you, the upside would be that wound severity is 'associated,' I guess you might say, with the magnitude of hp damage.
I'll come out and say it I'm 100% okay with going from 'still standing, not big deal' to 'help me I'm almost dead.' That is because I HATE death spirals.
You've got a problem, then, if you also want to inflict gruesome, impossible-to-heal-or-even-stabilize-overnight wounds.
DoaM is poorly written, poorly balanced, poorly explained, poorly related to HP and AC.
DoaM is mathematically identical to damage on a successful save. Any problem with DoaM would necessarily also be a problem with DoaSS. Since I've heard no complaints about the latter from people complaining about the former, it's clearly not the mechanic that's the issue.
I consider HP to require magic to heal and a good solid blow to remove.
This is at odds with the way hps are defined in all editions of D&D. You're free to change the rules, of course - and, as long as you're changing hps for your campaign, you can also change DoaM and or DoaSS to suit your new version of hps.
And yet that isn't really what DoaM is modeling. If it were then more archetypes would have it.
I get that there's a temptation to give anything the benighted fighter gets to everyone else, and I generally frown on that impulse, as it makes it hard to keep the fighter balanced and interesting, but, in this case, sure. DoaM is a mechanic that could be used to model a range of things. A cantrip might do damage on a miss, for instance, or quite a variety of monster attacks. The mechanic an also be used to consolidate attacks and saving throws into a single, simpler system. So it's certainly reasonable to think it can and should pop up in more places than just one fighter option.
No, DoaM has been described as a "blow so brutal that--" and that explanation does not fly with me.
Interesting, given that you say, above, that hps shoud take a solid blow to remove. That rationale would seem to fit even with your narrow and unsupported-in-any-version-of-D&D concept of hps.
No one else works that way. Dragons can smack you across the room - IF they hit. If they don't then surprisingly a 12000lb dragon doesn't deal secondary damage in he terms of his STR mod.
No reason some monsters couldn't do so. And, of course, every spell and monster attack (and 'powerful' but still at-will cantrip) that inflicts damage on a successful save /does/ work the 'same' way.