I want to talk about tempering

[Cilia]

Do primals know that the aether they need to exist is not unlimited? Can a primal exist on its own if it drains the world? Do they know they would eventually run out of energy and cease to exist or is there and amount of aether that can make their manifestation permanent... or do they just ignore the implications of their aether consumption because it doesn't mesh with their goals? With no aether and nobody left to feed them belief do they just cease to exist?

To an extent. It varies from primal to primal and probably depends on what their goals are. Ramuh, for instance, is as wise as the Sylphs imagined him to be - he is aware of the stress he puts on the environment, and as a consequence doesn't want to be summoned except in times of great danger and will allow himself to be banished back to the aetherial plane if his presence is unwarranted. (Like all primals he's still rather extreme in his mindset, but the point stands.) Bismarck, on the other hand, doesn't even appear to be sentient, so he's likely unconcerned with any of that. For your average primal (i.e. Ifrit, Titan, Garuda, etc.) they may be aware of the load they put on the environment, but consider fulfilling their summoned directive (for want of a better term) and tempering followers (as a means of self-preservation) more important.

A primal can't exist without consuming aether, so if one managed to drain the world dry... yeah, it would cease to exist. If all memory if it was scoured from the world, it would cease to exist as well; that's the point behind tempering. Even if you killed all of a given primal's tempered followers, that primal would still exist as long as someone remembered it and could, theoretically, be summoned by an untempered individual or group.

EDIT
Beat to the punch...