Hmmm. This is interesting. I still really, really value Sass, so what I have been doing is a bit of both:
Sass » CSS » PostCSS (cssnext) » CSS
This means we can use standardised syntax for variables (--foo: bar;) but still transpile that down to static CSS for old browsers. As soon as we can use custom properties everywhere, we can transition over more easily, but still leave other things to Sass.