I'd have to disagree with you on this part. While I don't know many US creators who've done this, a lot of mangakas started out with eromangas before they made their main series and projects. Just to name a few:
Koshi Rikdo (Excel Saga)
Rei Hiroe (Black Lagoon)
Kohta Hirano (Hellsing)
Masamune Shirow (Ghost In The Shell)
Ken Akamatsu (Love Hina)
It's not that uncommon as people believe it to be. Whether or not they enjoyed making NSFW content is another matter, but to say they've never gone on to do other things (especially their own projects) isn't really true.
I feel better (harsh) advice would be : If you're trying to make profit from your art passion, you need to learn how to market like a merchant. You have to give people reason to be as hyped about your story as you are -- and that does mean understanding what you like about your work and how that can appeal to other people. Ask for feedback: understand why your story may or may not be niche. And if it IS niche, figure out how to get other niche people to read it. You can't sit there and go "why isn't anyone buying" if you're not trying to sell your product. Remember Why and How:
"Why should I read your work and how does this benefit my time?"