Ask HN: How would you design a legacy-free OS?

If you could design an OS from scratch without having to take into consideration backwards compatibility or user-learned expectations of interface behavior, what would it look like?

Would we still use a desktop metaphor? Would we still have file paths? Etc.

Assume in this thought experiment 90%+ user adoption: other OSes would be too specialized for widespread use or would be vastly inferior. Also assume today's hardware/network technology (which makes this a little anachronistic because much of the hardware/network today has been influenced by OS design choices). This is sort of a remix/reboot what-if: What if you were back in the late 70's, early 80's and could set the table for how people used computers for the next 50 years (or more), what would it look like knowing what we know now about computer usage?