You need to clean slate the entire world because everything is interconnected. If , for example, you want files to be more than streams of bytes then you basically have to re-invent the Internet as well.
The older I get, the more I realize that we don't need a clean slate OS. If you pick any modern OS right now, the most annoying parts are never the parts that were from 30 years ago; it's almost always the newest thing.
You can't convert anything to anything at the boundary and still have full interoperability. An example is resource forks on classic MacOS -- they never played nice with an Internet that assumes that files are only a flat stream of bytes.
Google's Fuchsia is still the best bet for a genuinely new OS that may at least find a niche.
Capability-based microkernels have been exciting research projects for decades, and Google has apparently made one that achieves acceptable performance, which was a significant challenge.
Not the baggage of DOS or Unix... Something modern and sensible without insanity of either system.