At any rate, on the original subject; the 2-2-2-2 fight design is, unfortunately, essential to the game. Mechanics must be designed around the assumption that there are 2 melee, because even though there might not be, there always could be 2 melee. If you design fights around 1 melee, 3 ranged, then any comp that wanted to run 2 melee would have to deal with the fact that one of those melee will be severely limited by mechanical design, which would not be the case for a 3 ranged comp in a fight designed around 2 melee. Since casters are not impacted by being close or far from the boss, mechanics must always be designed around however many melee they can accommodate. If fights were designed around 4 melee and you could bring all 4 DPS, then people would just bring whichever 4 jobs are the best right now and you wouldn't even see things like aiming DPS in the current meta (which is a separate problem, but still unfortunately the case).

2 melee design is to allow both double caster and double melee to work--it doesn't demand one or the other.

Besides, in the current fight design of Dawntrail, you could argue that in all cases except damage, double melee isn't necessarily the best right now. It will always be the most damage, but now that Picto is out and doing damage comparable to and even surpassing melees, there's a strong argument for Melee + RDM/SMN + PCT + Aiming. This comp is damage suboptimal, but consider that throughout its entire life, RDM has been damage suboptimal, and it still sees a high playrate during progression and in party finder due to the value of the resurrection. If the upcoming Arcadion tier doesn't have tight DPS checks, I expect that the above comp will be the best in slot for progression or casual play.

The fact of the matter is, while 2 melee design exists, it is by no means necessary unless you are playing at the highest level, and even then, double caster can be very strong. Insisting on 2 melee is more of a community standard than a developer standard, and devs simply design mechanics to have 2 or more melee slots to ensur that, if there are two melee, they're not getting screwed over by mechanics. It's not to force you to bring two melee, and double caster works great right now.

Aiming are still in a super bad place, but there's less of them, so I guess that might be why they don't want double aiming (even though for 99% of content double aiming would still be fine).