My head-canon on the whole Eikon/Primal thing is that an Eikon is an ancient being of monstrous aetheric power, that has been powered to even greater strength through years of worship; seen as a God/Goddess in the eyes of their believers. They are an almost perfect representation of their original self, being of worship or concept... if not, then they are indeed the actual original thing. They may have been kind, non-malevolent as well, much like Ramuh is to his Sylphs, thus not regarded in fear by the people or tree people, etc etc of Meracydia. Don't need (from what we can tell) to be sustained due to its immense power and being an original being with its own original aetheric source.

Whereas a primal is a weaker being, a bastardization of the original thing, a version very different that is viewed in that way because of their followers. The thing might have been a thing in its previous, original self, or it is or was something different entirely (theory on Garuda being an Allagan scientist) but now they are projections of a belief and thus, don't represent the original in power nor appearance. Needs to be sustained, due to being nothing short of an aetheric conjuring of something.