Sure, and that's all it is. And a dropped enemy is only dropped until someone heals it. As long as healing is an available factor, even a dropped enemy has an expected future DPR. That's the oddity the thread calls 'whack-a-mole.'
If you know healing is a factor then sure, a dropped enemy may have future DPR. What I'm arguing is that in the majority of cases, monsters probably haven't had to deal with healing. When orcs raid goblins or human villages, the odds are they are not going to face a healer, much less a plethora of them. As such, it's unlikely that their tactics would revolve around something that they've rarely or never had to deal with before. In the rare event that they have to deal with a healer, "kill the healer" is probably a sufficient tactic. Coup de gracing everyone and everything simply leaves you open to attack and makes it more likely that members of your own party will be killed.
Which is a fair assumption. Given the availability of healing to PCs, OTOH, it's a fair assumption that they will get healed. Whether the rules that make that a reality are unique to PCs, rare beyond the PCs, or common is a matter of campaign style.
Granted.
You are certainly not alone in saying things like that. In classic D&D, a party couldn't really succeed - get through low level - without a healer, typically a cleric. So the assumption of PC healing was pretty nearly a given.
IME, that's not true. I've played in a number of 2e campaigns without any kind of priest in the party. You had to adjust your playstyle accordingly, but it was quite feasible.
Heck, in BD&D clerics didn't even get spells until 2nd level.
That wasn't really spelled out until later. In 1e, for instance, any human who wasn't '0 level' had a character class - probably fighter, but a class. The DMG presented fees for spellcasting as if casters of moderate or even high level were present in numbers in any city.
3e came out and said that PC classes were unusually - that most people were commoners, and most of the more exceptional ones were other NPC classes (but even one of those common NPC classes, the Adept, could heal).
4e was the only version of D&D to intimate that PCs might be nearly unique in possessing a lot of healing resources (Surges, second wind, leader powers).
1e was a bit vague, but it has this to say on DMG pg 35:
Human and half-orc characters suitable for level advancement are found at a ratio of 1 in 100. Other races have an incidence of 1 in 50. However, as most of these characters will be other than low level adventurers and already in a situation they are satisfied with... about 1 in 1000 population will be interested in offers of employment as a henchman.
So 1% to 2% of the population has a class, and of those only about 0.1% will be available for employment as henchmen (classed adventurers). Assuming an even distribution of classes (not reasonable in 1e, given that rogues and warriors were generally accepted as more common than priests and mages, but for the sake of argument...) we end up with about 0.025% of the population as adventuring healers. Given medieval population densities, that's not a lot of adventuring healers spread out across the world.
In 2e, pgs 17-19 discussed the rarity of the adventuring classes:
The great mass of humanity, elf-kind, the dwarven clans, and halflings are "0-level" (zero-level) characters.
It further discusses how the majority of wizards and priests (as well as most of the other classes) are not adventurers.
Certainly there were fees for spellcasters, but these were typically for non-adventuring spellcasters. The wizard who spends all his time locked in his tower with his books, or the high priest of a temple.
Either suicidal or insane would probably be quite sufficient. Villains and their henchlings in genre, and even fanatics of other stripes or grimmer heroes, are often willing to fight to the death and 'sell their lives dear' - which in the context of a D&D world with magical healing and raising, might very well mean 'making sure' of a fallen foe over possibly taking down a second for the few seconds he'll stay down before being healed.
I suppose we'll just have to differ on this point. That's not how I play the majority of my villains. They want to succeed, but even more than that, live to fight another day.
That's an adequate acknowledgement of the issue, I suppose.
Is it really desirable for PCs to have the ability to stand eachother up from mortal wounds, and for that to be the most efficient way of approaching in-combat healing (it maximizes hps restored because of the healing-from-zero rule) or might some DMs want to tweak the rules to make it a less optimal tactic (healing from negative, for instance)?
You can adjust campaign assumptions to minimize the impact that PC ability has on NPC actions - making PCs rare and NPCs generally ignorant of their abilities - and you have, which is fine, but it's just one way of partially dealing with the issue. A mechanical solution might be preferable to others.
I don't deny that some people might have a problem with it. I have yet to see it become an issue in my games, but I fully acknowledge that some players might abuse the system. Of the 5e campaigns I've run, there has always been a healer of some sort who could cast Healing Word, but the players don't generally play whack-a-mole. If they've been downed, they usually go on the defensive. Another party member will distract the monsters so that the injured PC can disengage and fall back.
However, I don't agree with it being the most effective tactic except from a purely theoretical point of view. There have been plenty of times when a PC was knocked to zero and missed his turn because the healer's initiative was after his. Or the healer healed him but then a monster attacked him and knocked him back to zero before he could even act. There are plenty of ways this approach can bite the party in the rear. When it works it can be very effective, but it's shortsighted and has the potential to go very wrong.
The only thing that I was arguing against was the idea that whack-a-mole necessitates coup-de-grace. From a metagame standpoint it is potentially the most effective tactic for killing a PC, but from an in-world perspective it relies on knowledge that the monsters may not have and tactics that are questionable in more common engagements. Even then, as I stated, my argument was predicated on the idea that PCs are rare.
If you want to discourage whack-a-mole healing, why not simply say that the first healing spell merely stabilizes the PC? If you want to be more generous, let level 3 or higher spells stabilize and heal. After all, Revivify is a 3rd level spell that can get you up from dead (at the very least, mostly dead).
If you want to really scare your players, use Chill Touch when their HP is low. They won't be able to heal for a round, so no whack-a-mole. You could invent a whole line of spells and magic items inspired by Chill Touch. Blue Slaad are good for similar reasons, since their Chaos Phage disease prevents the recovery of HP. Nailed the party fighter with that during a big fight last session; he was sweating until the druid cast Lesser Restoration (but that brought the druid into melee range so I was able to panic them again by focusing on the squishy druid).