Imagine being given the opportunity to design how OS X or Windows should look and feel from the ground up.
Linux is obviously another option, but unlike with TempleOS, you would not be treading new ground, and it would be hard to not just evolve the Linux interface, rather than defining it from scratch.
You can prove me wrong, of course, but why not just create a completely new UI on top of the Linux kernel? The Linux kernel is completely agnostic in regard to user interfaces.
Not that you cant create beautifully usable interfaces within such restrictive limitations, but minus all the spark and sizzle of modern UIs I think it would be a challenge to first get a UI designer _excited_ about creating a solution for TempleOS
The work of Susan Kare is held in high regard. I guess the pioneering aspect has gone and it'd be more like carving a pixel-art niche.
Having nothing to directly copy makes imitation harder, but it doesn't make avoiding imitation any easier.
Also, TempleOS _does_ have a user interface, possibly (I don't know TempleOS internals) technically even more so than Linux, where everything above terminal I/O is an add-on that can be replaced.