Why is the next new job a melee DPS?

[Vyrerus]

RDM and NIN are each very capable of filling in for either the melee or ranged slot.

Uhh, sure, they both have melee and ranged moves, but at that time especially, unless it came around in NIN's rotation that they were going to use Ten Chi Jin to trigger Suiton, they used it to do the combo ending in Doton. This meant more than ever, be in melee range, but it's not like a NIN is going to stand at 15y and Throwing Dagger for the whole fight. That'd be insane. A RDM standing in melee range the whole fight is much more believable, just reverse Displacement after melee combo if the timing of mechanics allows it.