While I'm not actually too sanguine about the idea of even a variant Extra Attack like the one above....
It kinda turns into an illusion of choice, though, since focus fire is such an obvious-best tactic under D&D hps.
Competence doesn't require Extra Attack, Proficiency reflects competence. Keeping DPR above the cantrip baseline of full casters requires Extra Attack - at the cost of significant resources. And, really, only one Warlord concept, the Bravura calls for more than mere competence, and one, the 'lazy' build, calls for the opposite (that is, I'd consider it to be a separate subclass, in 5e - I like 'Prince(ess),' but the genderless 'Icon' or something is probably a better name). It's worth noting that in 4e the 'lazy'lord and the Taclord used mostly the same features, but just had different stat arrangement, but, in 5e, it'd make more sense for them to be separate sub-classes.
I don't see the issue. It's not like an attack roll in D&D represents a single swing. If you're adjacent to two or more enemies in the course of the round, you've likely traded blows with each of them. Which one you hit logically shouldn't be entirely under your control, but, for convenience, and, perhaps, a teeny bit of player agency (though, again, mechanical effectiveness of focus-fire makes the choice of who to attack nearly illusory), it is left entirely up to the player.
The choice between attacking another opponent and granting an ally advantage against one you've already attacked isn't unreasonable or 'unrealistic' at all.