It's my understanding that Apple has, over the years, gradually supplanted the parts of Mach that really matter. For example, Apple no longer relies on the Mac

It's my understanding that Apple has, over the years, gradually supplanted the parts of Mach that really matter. For example, Apple no longer relies on the Mach port mechanism for IPC, instead mostly relying on their own, better XPC, which forms the foundation for libdispatch (aka Grand Central) and OS X's sandboxing. Of course, XNU is also a monolithic kernel, not a microkernel. Transitioning to a different kernel would probably be a costly mostly-zero-velocity project for Apple, especially now that they are no longer in the server OS space. A different kernel might require replacing IOKit, unless one could write a compatibility layer for it.