The vast majority of people don't run a Linus kernel. They run a distro kernel. Distros can (and do) ship multiple kernels with different sets of patches and options. They have a default, but they also have a default web server and C compiler.
The kernel is abused by all kinds of different workloads. Distros choosing to offer kernels with more "speculative" patches will have to support them. The kernel is a risk-averse environment. I think that's the reason, not fiat by Linus.
Also note, a good counterexample to your main point is the android kernel. Linus-kernel:Android-kernel is quite close to gcc:llvm.
Replacing big chunk of functionality takes a lot of resources, so it helps to be apple (LLVM) or google (android kernel).
Also, don't underestimate kernel modules either. If you want to, say, expose page table information that "linux doesn't let you", you can write a module to do so.
[edit - removed Con Kolivas related part of response. Don't want to drag up old flamewar.]