Ask anything to Sass TeamView other answers to this thread
Do you see CSS4 as the end of CSS preprocessors?
The way I think of it, Sass is only as powerful and CSS allows it to be. As CSS get more features and extensibility, Sass can leverage that to be a better abstraction.
If you look at the progression of the CSS specs, they honestly aren't focusing on increasing the complexity of the language and it's syntax. What they are instead focused on is what you can do with the selectors and properties you have. Think, Flexbox and all the lovely features we've been getting.
Further, there are things that just don't make sense for the browser to ever implement.... Sass (and other processors) do a lot of bundling, merging, splitting, importing, etc. Things that just simply make sense on the server side.
No matter how powerful the low-level OS language Assembly gets, your probably going to prefer to use a language one step up, so you can compile and optimize and handle all sorts of nice things we get from higher-level languages. CSS is the core of what's possible, and Sass is trying to make it powerful and easy from a coders perspective.