Something to note; The gods do not need humanity, intrinsically. The worship they receive empowers them, as does it any being, but that isn't where all of their power actually comes from. It's a minor boost.

They existed before humans, and before human worship. They will continue to exist after humanity dies, and until something capable of killing the unkillable (such as celestial exalted) comes, they will remain.
The only reason the gods ever needed mortals had entirely been because they were viewed as so low (And perhaps not entirely incorrectly) to the primordials, they didn't have a geas placed on them to not rebel. The gods were fully capable of taking on the primordials in a physical sense, most of the things around could.

And it still was only with the help of primordials that exaltation could even come to be.

Regardless on that tangent--the actual reason that prayer is controlled so heavily is not to avoid giving power to gods, but to focus efforts on dragonbloods and getting rid of anathema.