There were no takers.
The question of "but what can you actually do with it?" went unanswered by Urbit's own promoters, even when they were incentivized with real cash money to come up with an answer.
There was also no meaningful documentation at the time, which is why nobody else could complete it.
Also, what does "Urbit's formal semantics makes ships trivial to migrate" mean?
You'd run an Urbit instance (a planet) in the cloud (ideally, on three separate machines where a disk write succeeds if two out of three nodes receive it) and then run sub-instances (moons) on each of your personal computing devices (computers, phone, tablets). You could then interact with your planet and/or moons from your browser.
You'd do this because you want to run cloud-type services like photo sharing and (micro)blogging and such, but you _also_ want to do so through your own UI, not UI controlled by someone else whose interests are disaligned with yours, like Mark Zuckerberg's.
I can't imagine it'd replace standard OSs like Windows, macOS, iOS, and Android.
> Also, what does "Urbit's formal semantics makes ships trivial to migrate" mean?
If you want to move a ship (a directory containing your planet's data) from one (cloud?) computer to another, all you need to do is shut it down, scp it to the new location, and then start it back up again.