Why do you say that? That's what I don't understand. As far as I know, that's not true. You can run a Linux system without the udev daemon, or with one written by you.
But the thing is, you can't remove udev from the Linux kernel if you want to keep it calling Linux, as it'd be a fork.
But udev isn't in the Linux kernel! How can I remove it from there?!
A GNU/Linux system consists of several components. http://www.gnu.org/gnu/linux-and-gnu.html shows some examples.
If you remove udev from Linux, I see at least two problems. One of them is technical: programmers expect Linux to have udev, because that's what the Linux authors advertise. The other is legal: Linux is a trademark, and if you replace something and introduce some incompatibility, it's a bad idea to keep calling it Linux.
> But udev isn't in the Linux kernel! How can I remove it from there?!
I think here lies the misunderstanding, udev was previously in the kernel repository, now it isn't. Fair enough. But consider that GNU is also separated in a different repository, but it still comprises the GNU/Linux system as a whole. Imagine we could only download GCC (necessary to compile Linux) as part of Adobe Flash, don't you think I could say the same? That "we can't even build a distro anymore without downloading Adobe Flash"?
Yes, and udev would compose an hypothetical "Udev/Linux system". But Linux itself doesn't need udev.
GNU / GCC is different because you literally can't compile Linux without it. But if and when LLVMLinux succeeds, I don't think you could say the same.
Would it really work? I think you'd have to rewrite some parts of the kernel, but I may be wrong now, in the past or the future.
Anyways, we're arguing semantics here, it'd be better for all to separate udev from systemd. It's like Windows installers usually bundling the program with some crap, and I know most people don't appreciate that.