But with the downside of basically replicating a lot of what the CSS engine does in Javascript, further polluting the already stressed "main" thread in many JS

But with the downside of basically replicating a lot of what the CSS engine does in Javascript, further polluting the already stressed "main" thread in many JS applications. And doing it in a slower, less battery efficient, and less cacheable way. And in my opinion, a language like SASS isn't a "hack" but is a very specific and extremely powerful language which is dedicated to one use (styling) and therefore most styling-specific code is very natural with SASS, however with JS it gets more difficult (want to extend another style defined elsewhere? You kind of need to work with JS that wasn't really designed to make that an easy task, in SASS it's `@extend %other-class`)