Subjobs are one of those things, I'd argue, that have to be there from the game's launch with future ability additions planned around their presence. XIV is arguably past this point of implementation, with such an addition further justifying a complete revamp of all content to maintain a certain degree of balance. That may be happening soon with EW and the numbers squish, but I'd also argue that's an easier form of revamp since they're likely just going to be tweaking numbers downward with a % value based on tweaks within ilvl and not accommodating people suddenly having a bunch of abilities they didn't have before.
Of course, concerns over meta are also 100% valid. What if I'm a Red Mage that wants to be more melee oriented? Of the existing XIV jobs, what would you pick for that purpose? I don't see a whole lot of synergy in this regard, which further tells me that some jobs are going to be more flexible than others. What would be likely, just like in XI, is that RDMs would be forced to take up more WHM abilities and shift away from their presence as mediocre DPS to mediocre Healer. And sure, if that changed queue role attribution, some might even jump on that because lolDPSqueues, but it's a safe bet that everyone who wanted to play RDM as a DPS, even with more of a BLM focus, would have a harder time finding manually configured groups.
Having also played XI, I was probably one of the community's more notorious advocates for buffing RDM's melee game. The pervasive mantra was that they were supposed to be the jack of all trades, and my interpretation of that was always that with the right gear and sub, you could be a tank, healer, nuker, or meleer. Unfortunately, a lot of people didn't look too far past the fallacy of that moniker because to them, getting an invite as a much sought healer role invalidated any other concerns for job identity and some even fell into the trap of thinking RDM was a better healer than WHMs when the answer was more It's Complicated(tm) depending on encounter(s) and party competency. Still, I lobbied pretty hard for improvements to the job's melee gear, traits, food options, spells/abilities, and so on. Irksomely enough, a lot of those ideas wound up manifesting in other jobs like BLU and SCH while RDM third-rate in most everything. Even RDM's implied niche in enfeebling was at odds with so many mobs being immune or being able to quickly shed a debuff, with many debuffs RDM never even natively getting access to. Probably the only other job to have a comparative identity crisis, albeit on a lesser scale, would've been DRK. It's melee game was good, but the caster side was pretty much garbage. No one would ever take a caster DRK for endgame activities, and for a damn good reason. This didn't have to be the case, but Tanaka was pretty much too stubborn or too much of a chicken to really embrace the potential of hybrid classes, while some players would feed into the paranoia that if they were good, no one would play anything else. RDM under Matsui is probably better off these days, but I'd say the implied mediocrity of the Jack role is still there.