I understand that thinking but it's ignores reality. To do what you want you want means maintaining 2 codebases (even if just for sub-parts of a site). It's really easy to say "This specific page could be static" and you are right, it could, but it would mean having fallbacks for every JS interaction on the page (or removing them if the user has JS disabled). There simply aren't enough people who die on the no-JS hill to care about, especially since it means ongoing development maintenance, testing, design/UI work, and the list goes on.
GitLab is built on JS and renders a white screen without JS. Enabling JS at all taxes my Core 2 Duo machine, and opening GitLab to a few thousand line file (or worse yet, opening the pull request diff view) taxes my top-of-the-line Ryzen 5 5600X machine running Firefox. GitLab is just badly written.
Or you could server render the pages and hydrate them as needed which is something easy to do with NextJS, NuxtJS, Remix, Fresh, among other modern frameworks for developing with JavaScripts libs.
