Hussar
Legend
In case you've forgotten, it's entirely possible to miss with an attack against an unconscious character. Moreso in 4E than 2E or 3E, but it's still a waste of a turn that could otherwise be used to try and KO the healer. (It's also possible, in 2E or 3E, for an attack to not take someone from -2 to -10 in one go.)
No, my argument has always been that the rules of the game reflect the nature of the game world, which means they can be equally understood by anyone who lives in that world. If you're playing in a world which is represented as 2E with the Death's Door option, then anyone who has seen healing magic in action will know equally well that there's no point in finishing off a downed opponent, since it's a non-combatant either way (the only point in healing a downed ally was to prevent bleeding out; they couldn't contribute to the fight until they'd had serious rest and recovery).
If you're playing in a world which is represented as 5E with the default settings, then both sides are likely to know that you should finish off a downed opponent immediately if the other side still has a healer active. If the PCs make use of that option, but you play the NPCs as ignoring that option, then you aren't playing the NPCs to their intelligence; you're role-playing poorly, and that isn't fair to the players.
So, which do you do Sealorn? Do you gank fallen PC's? 2e had no recovery time after you went negative AFAIR, so, from 2e onward, so long as the healing brought you back above 1 HP, you were good to go. So, do you play that way? After all, you admit that not doing so is role playing poorly, so, I'd expect you to gank downed PC's every chance you got.
And, ok, let's run with the idea that the rules of the game reflect the nature of the game world. Are your settings entirely over run with undead? From at least 2e onward, anyone killed by a wraith becomes a wraith. By 3e, that's in a d4 rounds btw. If you start with a single wraith, and assume that every wraith kills 2 people per day, that's a billion wraiths in a single month. Why aren't all your game worlds completely destroyed by undead? Aren't you a good role player? Why aren't you role playing your undead NPC's?
On and on and on. D&D is not, nor has it ever been, a world building engine. Trying to use it as such means you have to ignore vast swaths of what the game actually says, and, at that point, well, you might as well not bother.