How does this mechanic affect Focus Fire, though? I agree it speeds up the fight, and makes the later rounds more deadly. But it still looks optimal to focus on one target at a time. Maybe even encourage you to focus lower AC first, because the bonus makes hitting higher AC more likely.
While its primary effect is on the "alpha strike," it has a secondary effect on focus-fire. Namely, you want to try to
size up opponents before you drop your heavy hits on them. Consequently, you need to poke at a few, which disperses some of the initial damage, which makes people more likely to consider hitting secondary targets over primary targets because those secondary targets have already lost some health and thus may be closer to death.
That said, if you want more spread-out damage, there is another relatively simple rule you can port over, but it would require you to use a 4e-ism. Specifically, minions. Minions are worth 1/4 the XP of a normal equivalent creature, but die the instant they are hit with any form of damage. (Technically speaking, minions didn't even have HP at all, but you could effectively treat them as being creatures with 1 HP.) Because they're worth only 1/4 the encounter budget of a full, regular creature, despite having all the same offensive and defensive capabilities as a regular creature,* you can field 4x as many of them. This
vastly encourages players to use area-of-effect attacks or to distribute their Extra Attack damage to multiple targets.
Alternatively, you can use the 13th Age "mook" rules, though those don't strictly deal with focus-fire either, because they have spillover from one creature to the next (that is, you use five "mooks" to be equivalent to one regular creature, each mook having one-fifth normal HP; swings that take down one mook but have leftovers carry over to the next mook.) You might be able to finagle it by having the carryover only work
once, e.g. you can cleave from Mook #1 to Mook #2, but nothing beyond that, or something of that nature.
*Technically minions were usually designed to be simpler than regular creatures, so they wouldn't have as many
active offense abilities. But in terms of raw stats they were equivalent, other than being "always dies in 1 hit."