We've been moving that way for quite some time, despite Linus's historic dismissal of microkernel ideas. Granular capabilities, FUSE, namespaces, cgroups, containers etc. all take us in that direction. Over the next several years, I think even the kernel grognards will start to see that the "center of gravity" for systems software has shifted to better languages and tools than are available in their sandbox.

There's been plenty of designs which are minimalistic, but haven't caught on for a variety of other reasons. See exokernels, for example. They're basically the minimum possible kernel.