something troubles me.

[Gasjockey]

You can play it more or less scarcely all you want, the end result will be the same, continual playing will breed familiarity, and familiarity significantly reduces your ability to be frightened by something

Yes, I agree. That is why they (Frictional) avoided using the overdone, clichéd zombie when deciding that they wanted humanoid enemies. It is less familiar.

And to the OP, what you're talking about is just this: familiarity. Still, Frictionals next game probably won't be on the level of most custom stories. Remember, most custom stories are made by 1-2 people (excluding voice actors) working on their free time, while Frictional is a professional (albeit small) gaming company with everyone working full-time jobs. And that's not even mentioning the fact that unless they decide to carbon copy Amnesia, it will be a whole new experience, different monsters, textures, sounds, story, everything. Playing Amnesia a lot might decrease your enjoyment of itself, but will probably have little effect on your enjoyment of their next game,