> build or render-time extraction of CSS

Render-time is waaaay too late. The ship has sailed at that point: so no, that doesn't take care of your first point.

> Many CSS-in-JS libraries also handle style modularization, often in ways arguably better than CSS preprocessors.

I've yet to see one that comes even close to SASS - not even the same game let alone the same league - so agree to disagree here.

> You should take a look at: https://github.com/MicheleBertoli/css-in-js

I have - all it shows is that css-in-js is not a realistic competition for sensible modular CSS via something like SASS (CSS Modules, etc, good patterns).