There’s a serious problem with this approach however- if you attempt to use any system that resolves combat as a matter of attrition, you will bring Single Target Focus into being.
That’s the core problem with attrition, it means that you can only win by slowly removing a resource of your opponent- and that means the best way to win is to focus all your methods of resource removal on one target until it’s gone
See, I think Single Target Focus is realistic. I would argue that throughout history a big chunk of tactics boils down to the offense trying to get multiple attackers on a defender and the defense trying to prevent that from happening. Concentrating fire works, whether it’s Greek hoplites wielding pikes so anyone approaching had to deal with two or three spears at once, to phalanxes trying extend their fronts or position themselves to outflank each other at the corners, to a wedge of cavalry charging a thin place in the line, to fighter jets maneuvering two-on-one in a dogfight. To the extent you want to spread attacks around, it’s to cover all the foes so they can’t do that, not because you can make good progress simultaneously everywhere.
Where a lot of RPG combat seems to me to be unrealistic in this respect is that players have supernaturally effective Command, Control, and Communications so they can always perfectly coordinate concentrating their fire, picking optimal targets based on how wounded they appear, switching targets as soon as they’ve downed a foe with no wasted effort, as if the players were a hive-mind. Early D&D dealt with this by having much more war-game-like rules for things like having to declare all your targets before any combat resolution, hard-and-fast zones of control so you couldn’t bypass enemy units, absolutely forbidding archery fire into melee or at point-blank range, strict resolution of the order of attacks so that all movement happened before any archery fire before any melee was resolved, making it difficult or impossible to disengage, and so on. Under these conditions, you were encouraged to balance concentration of fire against the chance of wasting attacks and the requirements and limitations of maneuvering.
There are “fixes” that you can make to encourage combatants to spread their attacks around, such as the ones Brian discusses in his post, e.g. a “Death Spiral” so that you can severely degrade somebody’s combat effectiveness well before you can eliminate them which encourages you to do that to as many foes as you can before you settle down to actually defeating them (Savage Worlds has something like this, with a Shaken result being sufficient to keep them from attacking while being a lot easier to achieve than eliminating them completely), but it’s not clear you really want to do that. Besides sometimes introducing their own problems, they strike me as addressing the symptom instead of the cause. You can encourage players to spread their attacks around instead of concentrating their fire, but they’re still going to be coordinating their actions with uncanny precision which to me is the real thing that makes the combat feel more like a game than a chaotic battle.
Mostly I live with it, because the cost of fixing the command and control issues seems to me to be too high in terms of limiting the players’ spontaneity, and my players (with perhaps one exception) are emphatically not interested in anything that resembles miniatures war-gaming. I sometimes rein in the table-talk to prevent them from spending excessive time coordinating what should be split-second decisions, but mostly allow them to fight like a well-oiled machine.