I dunno, the arguments around monolithic v microkernel have some good points on both sides. Also, Darwin is not what I'd call a microkernel. At best it has elements of both and is probably best described as a "hybrid", which may be no bad thing.
Actually I agree, the argument for and against either design is compelling either way! But from a philosophical view point I prefer microkernel... Though I fully understand the real world does not fit the ideological one

Yes, Hybrids are the best real world solutions in almost all situations!
Why, do they not do what you require?
A lot of these design arguments are very interesting but in the real world, if it isn't broken, why fix it?
Goodness me no, Linux meets and exceeds all requirements, but since I am running these boxes/boards as a hobby I would prefer to fit my ideology rather than practicality... I guess the same reason people still want to run PPC CPUs even though they offer no real world advantage.