"I don't see how anything else could possibly compare to that."
"work at decent performance and scale properly to SMP systems."
Again, we don't want to replace Linux. And yes, you can have decent performance and scale properly on SMP systems without comparing to Linux, i.e. not on 1000-core machines & such. BSD kernels achieve it well for their use for instance, and there are much less companies etc. than on Linux.
"there's no chance at all that an userspace solution with much less developers can compete"
Again, we don't want to replace Linux. There's no way we can achieve the same speed etc. So what? That's not the point. The fact is that even with much less developers, we actually got to the point that the Hurd works, has 68% Debian coverage with its nice installer and the nice translator feature.
"a monolithic kernel using a managed but compiled language like Java/C# is probably much faster than a multiprocess userspace microkernel system while having the same isolation and better security (no buffer overflows, ever!)"
No buffer overflows, right. That alone does not bring full security. Yes, there are other ways to do it. Now, please bring us the code. GNU/Hurd _has_ working code _now_.