As I'm thinking about this, the main thing that is left before I could imagine a build-free website/webapp is TypeScript support in the browser. Even just esbuild-style stripping of types would be enough, but I can't imagine bringing myself to write even a small amount of JavaScript without types any more.
Even without that, a tiny system that just depends on tsc should be within reach at this point.
I wouldn’t personally make these tradeoffs but for the sake of argument if you're willing to write substantially more type info into JSDocs (AI tooling may alleviate this nowadays) you can retain most of the coverage and assurances without using anything Typescript adds to the syntax superset while still using it for type checking during development. Even without the type-heavy JSDocs you might be able to alter your coding style to lean into easing and increasing inference (more classes instead of types or interfaces) and get pretty far.
Honestly though, I just never really run into use cases where I need to "make a page in a day" and where modern tooling gets in the way. I'm comfortable with create-react-app, NextJS, Gatsby, and a couple other things, I can definitely put a new static asset project from one of those on a VPS by shuffling files over scp in the timespan it takes to make a pot of coffee, and I would strongly prefer having those tools than trying to be "pure" for hand-wavy reasons and unproven benefit.
Even without that, a tiny system that just depends on tsc should be within reach at this point.