Or perhaps we have it backwards. Perhaps by committing it to writing they were trying to preserve the myth, and there by cover for Odin's identity.
I'm not so sure the Grendelkin/Genoskwa are trying to be forgotten. I think they want people to fear them. They believe they are meant to rule the Earth.
What's distinctly different about Forrest People as opposed to other supernatural types, is they are mortal. They're basically like an alternate mortal race to humans. I don't doubt they could summon Outsiders, for example. I don't doubt they're blood is so potent because they are both mortal and extremely magically attuned. But I suspect they're incredible strength, longevity, and magic is a trade off against mortal ingenuity, persistence, and a much higher birth rate. Humans were so vulnerable they had to breed lots and constantly adapt and evolve to survive. The Forrest People seemingly haven't adapted or evolved much if at all, since the Stone Age or earlier. Their birth rate I imagine was already low, but I imagine as humans have spread more it's probably decreased.
I am not so sure that the Forrest People need to rely on belief any more than humans do, both being mortal and not spirit beings. Hell, I suspect Bigfoot's have as much Free Will as humans their choices also branch reality - but I suspect they're a bit more aware of that. Which might mean they're also a lot more careful with their choices.