Good points, all. I especially appreciated those other artists you mentioned, though as I considered further, there were quite a few more I would add to the list. Perhaps surprise was an improper emotion. We've seen so many strong serialized artists come out of the Touhou scene or fall out of it, I suppose Yonurime's significant longevity separated them in my mind as an outlier. Given that I don't quite follow the scene anymore, this may too be inaccurate.
Writing new, original stuff is... hard and often punishing, especially when you have to pitch it to heartless, market-obsessed publishers instead of a warm community of adoring and supportive fans at conventions. There's also the merciless scheduling of serializations, the push to test well with major demographics, the demands and standards of editors, and a million other obstacles.
I would add, briefly, that publishing original content is gradually becoming easier. Speaking solely from the yuri community, publications like Galette, and anthology collections like Yuri Liquer offer significant, semi-traditional avenues. Then there are web publications, self-publishing through Pixiv, Twitter, or even monetized platforms like Booth. Of course, even if we assume a desire, there are surely some who would find none of the options available to them acceptable.
At least Yonurime's still producing manga as of last year
And of course, I'm forever grateful to any artist who sticks with it!