If there were other worshipers in Cyrodil (and perhaps elsewhere?), wouldn't it be likely that a group like the Mythic Dawn would seek out those other worshipers and say "Join or die?" Maybe over the course of a generation or two, the various cults have been forcibly melded into one? Given this wasn't just a few misguided souls, but rather the active movement of Dagon himself, one would think they would be at least mostly successful at such an endeavor.

When there is a true prophet about, those who do not flock to him are not very good cultists.

So the Mythic Dawn assimilated all other Dagonites, like the Cyrodiilic Vampires and the Imperials have done before them? Ugh, why is everything in Cyrodiil homogenized?