James Stoup has written a long-winded bit of speculative fiction about how, with the departure of Avi Tevanian from Apple, Mac OS X might be switched to the monolithic Linux kernel.
Of course, xnu, the hybrid kernel used in Mac OS X is derived from the mach microkernel but is not strictly a microkernel and has many monolithic kernel features.
The licensing of the Linux kernel is also a serious stumbling block.
I suppose it would be bad not to give the link of the article so I’ve added it in. Everyone’s a pundit.