What is Zodiark?

[Cilia]

Also, if all of the gods, Hydaelyn included, turn out to be primals, than the logical solution to all of the problems of FFXIV would be to simply put a stop to belief entirely... Which would probably require the mass genocide of all species, everywhere, throughout the cosmos... Admittedly, that would make for a very interesting plot twist, but I don't think we've seen any villains in FFXIV so far who have that kind of special brand of Kefka level insanity.

Not necessarily. That comes back to the reason for the primals' existence - because mortals want something they cannot easily get without divine assistance, or power beyond their means. Quoth Igeyorhm:

[Primals] are the embodiment of mortal will - of mortal desire.

... but that's not entirely true. Primals are the embodiment of mortal weaknesses, mortal flaws. People tend to create and summon primals in response to a hardship - that is, something they cannot overcome without great effort. The amalj'aa summoned Ifrit to help them conquer more of Thanalan, which Ul'dah's presence makes prohibitively difficult. The kobolds summoned Titan to protect them from Limsan expansion and reclaim the land they lost, which they cannot do without his assistance. The Ixal summoned Garuda to help them regain the ability to fly and destroy their enemies. Ysayle and Pope Thordan summoned Shiva and King Thordan, respectively, to end the Dragonsong War.

In all instances of a primal summoning thus far, it was due to tension between ideals and something being unobtainable without their power. Clashing ideals are one thing, but summoning a primal is not necessary to settle a dispute of ideals. The fact that people will resort to summoning a phantom god speaks volumes about their character, though - their weakness of heart gives birth to a desperate wish, and it is from that desperation a primal is summoned. That's what makes us so strong - not only are we able to overcome the despair that creates and summons primals, we never give in to despair ourselves. We fight with our own strength, and don't borrow power from or rely on the power of a (phantom) god. (The only time we really needed Hydaelyn's protection was to survive Ultima, which was, well, a freakin' nuke.)

If everyone was strong enough to accept hardship, there would be no need for primals. Faith is not a bad thing, but when it goes to extremes like the primals do, well, conflict is inevitable, even if it is to the detriment of everyone involved. Thus, while the Garleans have a valid point, they simply take it too far because their extremism only pushes others to the opposite extreme.

Any social commentary on religion is purely imagined on your part. I assure you.

I admit it'd be kinda boring to have every god be a primal of varying power, or perhaps an eikon (which as far as we can figure is just a primal that doesn't need active worshipers). Still, we have yet to understand much beyond "Hydaelyn is a goddess." As a scientific person, it's not really in my nature to just accept that as truth, or not ask where she came from or what gives her power...

I try to avoid taking my cues from previous games in the franchise, as while elements tend to be common I personally don't think entire plot points and themes should be reused. (Makes them seem lazy.) Most of my cues are taken from the Dark Souls games, which despite being 10x more cynical than Final Fantasy also deals with cyclical history, an unbreakable circle of fate, and duality, like XIV seems to.