Valid point, although I'm surprised that a niche has STAYED a niche for 20 years.
Also, I suspect that many businesses would sell more if they added some marketing.
For example if you are making $150K/ year and spending 1600h on programming and 400h on marketing and you doubled marketing to 800h and doubled revenue then that's time well spent. (and you are at 33% spent on marketing).
Your situation may be very different.
However, most programmers I know don't enjoy the marketing, so they don't do it. That may not be financially lucrative, but that's their choice and it's fine.