I'd argue that from the perspective of an operating system vetting security, a good microkernel implementation with userspace drivers would be a great win. Even when you can vet the entire linux code base (for the most part) and use only the free drivers, you still have multiple attack vectors available to possibly get code running in kernel space. Which is bad.
A lot of people (myself included) probably don't think Linux is the last kernel to rule them all, ever. It doesn't distribute itself well, it is quite large, and its speed is derived from its massive developer staff. Simplifying your kernel doesn't really matter when it has hundreds of active contributors, but what about in a hundred years when all the current Linux devs are gone, and you have to try to get new generations to adopt the code base and maintain it?
A lot of people (myself included) probably don't think Linux is the last kernel to rule them all, ever. It doesn't distribute itself well, it is quite large, and its speed is derived from its massive developer staff. Simplifying your kernel doesn't really matter when it has hundreds of active contributors, but what about in a hundred years when all the current Linux devs are gone, and you have to try to get new generations to adopt the code base and maintain it?