Well, you don't need to rewrite all of Linux. What's currently happening is that some subsystems are incrementally being made more accessible from a user contex

Well, you don't need to rewrite all of Linux. What's currently happening is that some subsystems are incrementally being made more accessible from a user context - e.g. a new flag for madvise(2) to make page fault handling in userspace easier, NSUSE to access certain network drivers as userspace libraries, FUSE to cooperate with the in-kernel VFS to get file systems as user processes, virtual consoles outside the kernel and so on. The demand for microkernel-like features is there, and Linux will progressively be refactored into a hybrid kernel with a standard set of "servers" you plug on top to augment it (systemd appears to be taking the role of the servers in question). Whether this architecture will end up desirable or an awkward worst-of-both-worlds combination remains unclear.