CapnZapp
Legend
Again, lots of words when it is so much easier to simply rescind the design decision to go with "no negative hit points".1. The logical course of action is not the only course taken. Enemies can be stupid, single minded, arrogant, honorable etc.
2. If the group of enemies have a different objective (survive, kill the PCs, kill one specific PC, get some loot, delay the PCs, capture the PCs, die a glorious death in battle or something else), then they will take different courses of action.
3. Enemies do not have perfect information. Without perfect information you cannot always execute the logical course of action, especially against a group of foes as diverse as adventurers. Adventurers get advantage of more-or-less knowing how tough their foes are simply by looking at them and having previously encountered them, monsters typically do not.
4. The tactical situation will dictate what is logical. If that last monster only has the option (because of movement, or crowd control or whatever) of attacking the AC 25 paladin with disadvantage, then maybe have him stab the fallen guy instead.
Now, I don't know about you, but I think that if a campaign is full of identical groups of monsters, consistantly with the same tactical situation, the same disposition, the same goals and all acting on perfect information, that's going to get tedious.
Also, you're right. If you've got a group of goblins, why would they keep playing whack a mole? Really, why would they? Are they just aware that their role is to suck and then die?
Whack a mole is only beneficial if someone who is downed is 'safe'. The penalties for misjudging that are dire. All you have to do is not make it a guaranteed state of affairs, and whack a mole becomes a losing game.
All this discussion and not a single word on upsides with that decision. So why stick to it, when it forces you, if not to change monster behavior at your table, to write long forum posts...?
I guess I'm asking you to reevaluate if the RAW really is worth defending that much.