Does plan9 support 15 different archs with all of their quirks and subarchitectures? WiFi stack? Support for different power modes? Framebuffers? Kernel probes, high resolution timers, extended inbuilt security APIs?

The beauty of plan9 comes from the interface to the userland, not from implementation details and features. If you wanted to add al those features to plan9 I seriously doubt you could get it as lean and quick as Linux.