In fairness, the hard part is scaling redundant HA phone operators. I'm sure between twilio, deep learning, and GPT3 they have the problem almost solved.
Riiing. Hi, I'd like to order tamale maker TK156.
No problem! Where in Tokelau would you like us to send 156 tamales?
I mean yeah, plain html/css scales well for any site that just exists to serve a couple lines of text and images. I can't think of a single site (outside of maybe craigslist) that can get by with that functionality today.
Newspaper sites are the obvious example you'd think of... but they need comments, advertising (or login functionality), moderation tools, etc.
All of the features you mention can be done without JS using server side rendering.
It would of course be more convenient to use JS for advertisements, but logins and comments used to be implemented without using JS for many pages, even if JS was available.
HN is an example of such a site. This site does not have scaling issues due to login not using JS.
Riiing. Hi, I'd like to order tamale maker TK156.
No problem! Where in Tokelau would you like us to send 156 tamales?