It's a strange niche to be in because it's something people are looking for but there was basically nobody doing it when I started out. It goes squarely against the two most fundamental pieces of business advice you can get: "Never build a business based on a missing feature of a popular software product", and "If there's no competition in the space, be warned: there's probably no market there".
Evidently, there are actually a few good left ideas out there that nobody ever thought of doing before.
First time I see this, is it really a fundamental piece of business advice?
You want to build a business, not a feature. Unless you can come up with more features, it's difficult to grow the business.
Then of course there is the risk that your feature will come standard in the next version of "popular software product." If your audience is niche, then you are likely safe, but if your feature appeals to all users of the product, it's inevitable that it will eventually be included in a future version of the product. You end up doing the R&D for free.