Except in real life there is no such things as healing word, regeneration and other shenanigans. You must take these into account. In a fantasy world, they exist and tactics will be adjusted accordingly. When you know that if you don't "finish" the fallen enemy that his friends can and will bring him back with a fury, you don't take any chances. You "finish" the job.
Just like your players will torch a troll, the hobgoblins will remove a threat permanently. I do not see this as punishing but as logical. Hey, even a wyvern might get off with a fallen one to eat the poor sod in its nest. Why fight to the death when you already have your tasty snack?
I think that presents a totally unrealistic take on how common that kind of magic is, and worse it's definitely metagaming of a clear and undeniable nature, because none of these monsters/NPCs know that it takes 3 hits doing any amount of damage, to finish a PC and prevent them from getting up again. So you either have to have a situation where NPCs frequently make mistakes, because they don't know how many hits it actually takes to prevent healing etc., sometimes hitting too few, sometimes hitting too many times, if you're staying in the setting, or you have to have them basically not do it and avoid opening the can of worms.
If you keep having NPCs go around and neatly use the correct number of hits to use up the death saves, that's pure, uncut, 100% metagaming.
There's no non-magical way a normal NPC or monster can tell when a player is so injured that they can't be brought up by Healing Word or the like. They don't know what death saves are. They don't know that any three sources of damage will do. You seem to going with PCs laying there moaning or something, but they're not - they're unconscious at 0HP. So if someone wants to lay into them, they can, but they can't tell when they're done unless it's crazy overkill.
What I suspect you're thinking is fine is a situation like, you have six guards around a downed PC, you decide to kill him, first three guards attack and hit, and because you, metagaming, know he's dead, the other three move off and attack other targets.
It's a ridiculously metagame situation. In the fiction, one guard would likely stab him, and the rest would move off (or none would). In an earlier edition, one guard would CdG, which would make sense. Not so in 5E though.
Anything can be made to make sense in the fiction. Just tack on whatever explanation is needed. That's the beauty of a fantasy game.
Hahahahaha no. That's just not even arguably true, and a great way to casually destroy the fiction.