I generally stick to PDF myself, but I do sometimes wish it would be more ergonomic to reflow a 2-column paper for reading on mobile on the go, for example. Also, ePub is easier to read in night mode than PDF recoloring, and seems easier to search through (try searching for a Greek letter in a PDF…).
EDIT: How is the math support in ePub though? Are people embedding KaTeX/MathJax or just relying on MathML, and how is the quality compared to TeX?
I'm guessing there are few web pages of any significance which need to stay exactly the same for a long time. Here is one example which I've seen trotted out from time to time on HN:
https://www.dolekemp96.org/main.htm
This is clearly the exception. It seems that maintainers of web pages usually expect that they'll need to maintain and update them for as long as they want them to be accessible, and that's definitely not something I'd care to do for research papers.