I agree, tools where devs are the primary/only audience might be more fitting for bundling with the kernel for longer periods of time. I don't really see KVM that way, though.
The problem with not leaving is obvious; there's a never-ending growth of semi-random packages distributed with the kernel, and pretty soon they'll call it LinuxBSD because it includes the whole OS.
Making it explicit that the kernel tree is an incubator rather than a permanent home means that it's not unbounded growth but instead more of an adolescence for new userspace code, which will eventually move out and go to college instead of sucking your resources forever.