I think CSS will go the same route of JavaScript. More developers are now writing code in ES2015/ES6 but compelling features will continue to come in which we will keep having to compile back down for browsers with Babel or similar tools. While I still use Sass/Less in some projects for legacy reasons, I tend to use cssnext (compiled with PostCSS) on new projects because it supports the upcoming standard features of CSS. For the ones that don't exist, I just add in as a PostCSS plugin or re-think whether I really should be adding another layer of complexity.
However, while cssnext/PostCSS is gaining popularity, I think that the majority of developers have just gotten comfortable with Sass and will continue to use it for a long time. Keep in mind that Bootstrap 4 is using Sass so a lot of people will use it just for that reason, although Mark Otto has expressed his interest in using PostCSS for v5.