I'd venture to say that Plan 9 was never meant to be a mainstream OS. It essentially served as a playpen for new OS/systems ideas and research at Bell Labs at the time.

A lot of those ideas were pretty influential: union mounts, per-process namespaces, user-space file systems, UTF8, human-readable protocols, rfork(), file-object interface ubiquity, concurrent programming model, etc. You can see how these ideas have pervaded our current systems landscape.

The implementation of Plan 9 itself is great, but I don't think success of the Plan 9 project should be limited to whether or not its specific implementation stays current and modern. The UX was never designed with the common user in-mind, and it arguably was never the core focus for the system. A greater success would be to see its actual core ideas spread out into other mainstream systems.

So instead of making the Plan 9 UX more modern, why not work on getting some of its missing features into Linux instead? Last time I checked, Linux still needs a union-mount implementation. This way, the world benefits.

BTW, you might like this: http://swtch.com/plan9port/