I think React is an interesting thing to think about here. There was a bunch of libraries that got ever-more-fancy about doing DOM manipulation. Those libraries helped manage the complexity of the model of doing in-place DOM manipulation. React chose something different – much more like the templates of yore – and instead of managing the complexity it introduced a simpler model that eliminated the complexity. That made it really different.

SASS/etc help manage the complexity of CSS, but they don't simplify anything at all. I think there's a scope problem here: the styling and layout of an application isn't just a bunch of selectors and styles in CSS. It's the class names and structure in the code, it's the implied connections between the expected rendered layout, it's a private language of expected elements and their semantics. You can't simplify that just using CSS. We're still waiting on someone to come up with a combined approach that actually works. I personally eagerly await someone's genius idea!