Laurefindel
Legend
I do have a sci-fi game (that I based on The One Ring's system) where PCs can only occupy so many enemies before getting swamped. There are actions you can take to occupy multiple enemies (cover/suppression fire, overwatch and whatnot) but otherwise "unassigned" enemies are free to do as they please, including blocking the way or finding an unobstructed line-of sight to shoot you point blank.The problem there is that just means any time the PCs are outnumbered (which means, 90% of the time), even if the party divides up, they'll be dealing with loads of monsters getting Focus and nothing they can really do about. Also if it applies to PCs you handed yet another advantage to casters and ranged, who will get it pretty often! I guess you're envisioning an "equal numbers" scenario, but that's actually pretty rare in my experience. It would also be a ton of extra book-keeping.
Conceptually it's not a bad idea to be clear, it just applies poorly to D&D 5E as a generic rule.
I do think there is something in it, like, it would definitely make the game more about spraying around abilities to keep enemies on their toes and so on, I just think it's going to work poorly when most groups are 4-5 PCs, 2-3 of which may be melee, who are often facing 8+ monsters.
In D&D, that would translate in unassigned enemies having advantage on attack rolls. It's easy to do there because the system has a baked in semi-theatre-of-the-mind semi-tactical-board where players and enemies are matched up, but it could work in D&D given on how this "unassigned" condition is granted.