when regular css can do a lot of what preprocessors can do, that's the day they will become irrelevant. But what is actually kind of a good thing and I think a lot of people use is the @import separation of files by sections, pages or modules and that's a pretty big thing. It's a lot easier to try and find a style for your header in a single file named header than to find a style in a 4,000 lines length file that contains everything.
I know it requires additional time to configure preprocessors but that's just once in the computer and then all you have to do is setup a file or open your console and install the package or just run the listener to compile. The errors can be managed with npackages that finds the errors and indicates you where it is and not seeing something like line 5,000 error.
Also a big advantage that preprocessors have are mixins, you can actually forget about them and on compilation adding them automatically, that's pretty cool and I really hate when I can't do that. Also you can configure it to minimize at compilation and that's something really cool.
When regular CSS can do all of that without adding extra loading files on production with @import preprocessors will die but I think perhaps in another 10 years we will be near that point but preprocessors will make updates and be on the lead again.