As i have come to understand there is one successful such kernel out there, QNX. And that while both the OSX/iOS and Windows NT kernel started out as a micro design, both Apple and Microsoft have been moving things in and out of the kernel proper as they try to balance performance and stability (Most famously with Windows, the graphics sub-system).
The OS was cautiously courting a "shared source" model where you could agree to a fairly permissive (but not categorically pure-open-source) license and get access to quite a few components' source code.
It was anybody's guess what might develop from that, and an intriguing and hopeful time.
And then BlackBerry came along and bought QNX and killed the shared source initiative. Really mad at BB for deciding to do that.
Nowadays QNX is no longer self-hosting - no more of that cool/characteristic Neutrino GUI anymore :(