[moonchild]

> Aside from performance, in my opinion a better API would also be transactional, at the filesystem level

A better API shouldn't be a filesystem. As a strictly hierarchical system, its only virtue is that it should be easy to create sandboxes and reason about the resultant guarantees. Emphasis on 'should'; in practice, chroots are easy but very ineffective. (I believe fuchsia[0] and plan9[1] do a slightly better job of this.)

Systems based (for example) on tags and tag categories are faster, more expressive, easier to reason about, and can be constructed into arbitrary trees on the fly should the need arise. (They are thus also capable of presenting a hierarchical interface to a legacy application, so compatibility isn't an issue.)

0. https://fuchsia.googlesource.com/docs/+/dart-docs/dotdot.md

1. Because plan9 does a better job than unix of making all resources files, inability to access some subset of files is a stronger guarantee.