After those the major use-case for JS preprocessors will for most use-cases only be bundling and minification.
Just look at the history of 6to5, babel, pony/polyfills, different build systems, different module systems, multiple different syntaxes like coffeescript, multiple different type systems. IMO we are much closer to the browser now and hopefully close to getting "normal developers" to write code that runs verbatim in a "normal browser". The outliers will always be there (Svelte, elm, etc.) but for mainstream code I hope to get back to shipping the code you wrote.
I hope that we get more features from TC39 after that. I think generators and asyncIterators are underused currently. Things like pattern matching sounds interesting to get into the JS ecosystem.
I wouldn't need Sass if mixins were implemented in CSS.
It might not eliminate it for everyone, but it will for those who care.
Maybe you still use PostCSS for some auto prefixing/ backwards compat stuff but the need for that should go away as the usage numbers for unsupported browsers go down.