One thing that's interesting is that, while microkernels haven't really won, Xen-style paravirtualization, which in a way is an even more radical version of the de-monolithization idea [1], has taken over in a lot of niches, despite performance being an even bigger problem, especially without hardware assistance.

[1] This paper, for example, argues that VMMs are "microkernels done right": http://static.usenix.org/event/hotos05/final_papers/full_pap...