The obvious superiority of microkernels is seen in the invisible ubiquity of QNX and L4 throughout the industry. There's probably more L4 deployments than Linux, in hardware like baseband processors.
The most common OS then is probably not Linux, but TRON. It's real-time, but I'm not sure if it's a u-kernel. Nonetheless, RTOS in general is well served by u-kernels.
Supporting your point, OK Labs claimed years ago their OKL4 microkernel had hit a billion units in phone market. Mainly for baseband isolation and legacy code (eg brew). So, it's exceeded Linux servers and slowly catching up to Android esp as it runs it virtualized.
And Samsung Galaxies use INTEGRITY Multivisor for Knox. Blackberry plus automotive often do QNX. Apple does a hybrid with Mach and wants a real one. A lot of design moving one way in particular. ;)
Apple does a hybrid with Mach and wants a real one.
I know that XNU is based on a chimera between OSF Mach and FreeBSD, but I'm pretty sure most practical u-kernel gains are lost in this process. They do have the basic resources like tasks, threads, VM and IPC with its system of port rights checking, but it's mostly leveraged as a convenient abstraction at best. I don't think OS X even supports per-application default memory managers, does it?
However, what you said about "wants a real one" really piqued my interest. Is it true Apple is researching pure microkernel designs for their products? That sounds great, do share some links.
Yeah, that and some basic functions are all they use it for. A write up way back indicated they though Mach was a mistake. So, the alternatives were a better microkernel or full monolithic. One unofficial project tried to port it to L4. Not sure what Apple's stance is under Tim Cook, though.
The most common OS then is probably not Linux, but TRON. It's real-time, but I'm not sure if it's a u-kernel. Nonetheless, RTOS in general is well served by u-kernels.