So we now live in a world where both monolithic and "microkernel" (hybrid) systems are commonplace - there's a bit shy of a billion devices running linux (android) and no shortage of windows and osx hosts around. What can we learn from this?

Well, it doesn't really seem to matter whether you use a monolithic or microkernel design. They both work well enough. Driver development is hard for reasons that have got nothing to do with this, and doesn't appear to change much between the platforms. Success/popularity of operating systems appears to be determined by other factors.

I'm not sure why people feel this is still worth debating. All the evidence points to it being completely unimportant.