One good way to explore the issue with subjobs is to look at RPG theorycrafting in general and how it evaluates different build options.

Optimal Vs Non-optimal is a really binary way to look at it. It is actually a gradiant from 'optimal' to 'suboptimal but viable' to 'non-viable.' For example, for a long time Samurai was suboptimal but viable, you didn't really want a Samurai in Stormblood, but having one wasn't gunna wreck your party for most content.

XIV works really hard to make sure everyone is in the 'viable' category. But there are also sub-divisions in that category. For example, "Ice Mages" are viable in most content, but so sub-optimal it drags the experience for everyone else. Heck, technically you can clear most content with the DPS doing literally nothing.

The issue with 'Subjob' systems ala XI or say... PSO2 is that they tend to not really encourage creativity, but instead create a system where you take buffs to the primary job. You see this a lot in P&P RPGs that have multiclassing too: It is hard to synthesize the advantages of say... being a monk and being a wizard in D&D without a specialized seperate class dedicated too it. It is too easy for a given job+subjob combo to be thematic but terrible. Ever see a Summoner-Techer in PSO2? Sounds cool, your a wizard who summons things and heals and buffs your party but in reality it is the very non-thematic Summoner-Fighter all day every day to get those generic attack boosts for your pet.

This is why games that allow creative builds tend to be way more open ended. PoE comes to mind as a game with a strong diverse build community with lots of competitive and viable options. This is because it creates a lot of 'fuzzieness' around what is optimal, based on what you value, your gear access, how early in the expansion your making that build, ect. That would be VERY hard to get into XIV, most of the time you would just take a subjob with a self buff that synergizes with your class, and while its possible a sub-optimal combo wouldn't make people kick you out of casual content it probably just wouldn't be as interesting as you think. That isn't to say its impossible, but it would be a design nightmare.

Subjob systems just sorta 'default' to wanting to have the secondary job purely exist to support the first, rather than being a creative synthesis. Breaking free of that is legitimately very hard.

I think the closest we could get would be 'alt jobs' like say... Healer RDM, or DPS DRK: Same job but its core rotation altered in goal, but with what they said about SMN and SCH I highly doubt that will ever happen.