Do we need some new terminology? There does not seem to be a word for the kind of thing Hurd is. "Replacement for the Unix kernel" seems a bit wordy. It's a thing that does what ordinary folk expect a kernel to do. So technically, it's a kind of virtual kernel, I suppose, but that would be far too easy to misunderstand.
On a completely different note, just what is it about some software projects that stretches them out over such huge time scales? Hurd, Perl 6, LaTeX 3, etc.
Well, I said it's a virtual kernel. That's virtual in the sense of "same interface, different implementation". A virtual X is not an X; it's just handled like an X. A virtual file is not a file. Virtual memory is not memory. Virtual reality is not reality. Etc.
Since it is not a kernel, but is supposed to replace one, virtual kernel would seem to be an accurate term. However, it's too easy to misunderstand, due largely to horribly incorrect usages like "virtual bank" and "virtual classroom" that have been floating around for a while. Alas!
On a completely different note, just what is it about some software projects that stretches them out over such huge time scales? Hurd, Perl 6, LaTeX 3, etc.