If it's just equations (i.e. using no TeX features besides the $equation syntax$), that's what their conversion workflow is for. You can submit your manuscript in simple TeX and then they convert it to their preferred format with some scripts. I believe they'll also accept Markdown extended with TeX-syntax equations (not sure if that's official, but I know one author who submitted a Markdown manuscript w/ TeX equations... though he got the proof copies back as Word, post-conversion).

Ah, now I understand: When you wrote "In book publishing it's shrunk to a very small niche, even in technical areas", you mean the "technical areas" that don't use equations in their books.

My background is blinding me to the fact that books with equations is really a small niche market. That small niche is completely dominated by LaTeX.