Niche stuff has always been economically viable. Niche stuff also tends to get publisher support. Realistically these days the only reason why books don't have an audiobook format is due to not wanting to support Amazon/Apple, or because they just don't want it. Voice actors literally are not expensive for audiobooks. You can absolutely afford one if your tiny $9 book hits more than 200 copies.

Unless you're talking about illegal niche, in which fair but I highly doubt stores are going to accept those. All generation helps with is content that will be free.