I like the idea of retaining preprocessors for nesting and mixins to write good CSS quickly, but also having the postprocessor at the other end to add polyfills where necessary.

This way, I can write SASS to target modern browsers and not worry about backward compatibility so much, because I know the postprocessor is there to catch the older browsers. e.g.: I don't want to worry about specifying a solid block colour for CSS gradients, I just want to specify a gradient.

But yes, I agree the implication that this could replace preprocessors is unrealistic.