Is there demand for this in the job market? Like, if you'd go work somewhere and not freelance, are there places where you wouldn't have to learn to work with Adobe Photoshop® and such?
The idea of exclusively using only a few applications definitely seems limiting. I understand in this case it's probably because of licensing and morals, but I can't imagine many businesses caring much about your principles on software licenses when everyone's passing around their ".sketch" files.
As far as I've experienced working only with floss, none of my clients ever cared about what tools I was using. Not only that but I haven't found much community that share/support/talk about running a creative business on floss only. But now I have hone my skills and I can produce the same results for the market/product I am working with, and enjoy the stability and flexibility of linux.