No where in game does it state "basic composition is 1 ranged, 1 caster and 2 melee DPS"... In most places it just says "(2 tanks, 2 healers, 4 DPS)" in brackets for most 8-man DF instances. In the few cases it DOES specify, it is 1 melee, 1 ranged, 1 caster and 1 "other" that is a duplicate of the three. The "meta" favored 2 melees because in ARR the highest DPS and utility lied within the melee role (MNK and NIN). In HW "meta" dropped off the caster in favore of 2 ranged to buff the highest DPS which lied, again, in the melee role (DRG+NIN). In Stormblood, the highest DPS has been in the caster role since 4.1, but people have been so used to their meta that didn't want to break from it. FFLogs recently (since 4.4) started taking double caster comps that are closing in for the fastest kills.
Here's the source from game to the party composition:
Now we have 6 "ranged" DPS to choose from and only 4 melee.. It's high time we see more variety over this "double melee meta" that I've personally grown very bored of.