I think we also experience a not-insignificant number of posters who do not actively play the D&D game they are actively complaining about. Yet its hard to parse that out since they don't necessarily identify themselves as such.
Those other communities just don't seem to draw D&D fans intent on yucking their yum in the same way. Maybe its just the size of the flame that draws the most moths? And some moths are happy moths and some, not so much.
I lot of those games are specifically marketed to D&D players that are disillusioned with the game, rather than people new to TTRGs. As such, people who have moved to another game are people that that game suits better than D&D did, and are thus not only happier, but aware that worse exists. That's a recipe for satisfaction.
On any gaming forum beyond a certain size there are people who doomsay, criticise the creators, or actively try to sabotage it. Its not unique to D&D at all: The bigger the company, the more it and its products are acceptable targets.
Yet when NPC adventurers of the same capabilities are the PCs' foes those NPCs should be easy to kill?
No. What works for one works for the other.
Applicable vs. PC and NPC alike, sure. This is one really good 3e idea that subsequent editions abandoned in their quest to have nothing ever bypass hit points.
The only problem with it as written is there should be something in there about familiar anatomy. As in, you probably can't coup-de-grace an ooze or a purple worm even if you've rendered it helpless as you don't know where best to hit it.
IMO death spirals should very much be a thing. The most important hit point to preserve shouldn't be the last one you lose, it should be the first one you lose: going from full to one less than full should have impact.
While that might scratch a specific "realism" itch, I believe that these mechanics were not included in the game very deliberately.
If you think that D&D has an issue with people going nova, or just deleting monsters with "save or die" equivalent spells, I think that it is a given that these will become even worse if these two concepts were introduced.
If it is reasonably possible to bypass a monster's hit points, then manufacturing a situation where you can do so will become the optimal strategy for killing them.
Likewise introducing death spiral rules pushes parties into the nova strategy even harder, as every combat becomes a race to push the other side further down the death spiral before they get the chance to respond and start pushing the party into theirs. The party will be even further incentivised to ensure that the DM gets to actually act as little as possible.
The emphasis on ambushes and surprise attacks that this would lead to was just viewed as a tactical exercise when a lot of D&D was about exterminating your way through an area, killing the inhabitants as you come across them. However the general slant of D&D has changed, and many groups will try to talk before resorting to combat, which is likely to be fewer and more difficult set-piece encounters rather than lots of more minor ones degrading the party through attrition.