You're comparing real world working implementations based on mathml with your hopeful expectations of how you wish katex could work in an epub implementation.
It's good to have hopes and dreams and a positive outlook, but your perceived solution does not exist. It's hardly relevant to present it as a better alternative.
No hopes and dreams needed, just use PDF. But making epub work properly for math is not a problem in principle, just make katex work in an epub reader, problem solved.
And people who are satisfied with substandard solutions are actually part of the problem, not the solution.
"Just write a full JavaScript and DOM implementation for your resource-constrained e-ink device"
Not that anyone actually follows the standard…
For the vast majority of fiction a format constrained to bold, italicize, paragraph break, chapter heading, and chapter break allows a far superior user experience.
[1] https://magnoliamedianetwork.com/ebook-publishing-platforms/
Kindle books generally start their life as ePubs that Amazon then converts to their formats before they list them. They don't even let you publish in mobi anymore.
Kindle's lack of MathML support actually makes the user experience worse for everyone. Publishers often make 1 ePub they submit to multiple stores so rather than make a MathML version and a shitty image version for Kindle they just make a shitty image version and now you have a math textbook that's larger than the PDF would have been AND the equation text isn't resizable defeating the whole point of a reflowable ebook.
Morally speaking they start as Kindle ePubs that support a very very limited subset of the ePub standard.