"I am a true Apple customer, I use Apple since they switched to OS X and I sure know what is possible with Darwin"
OK.. Well.. I've been using Apple on and off since they released System 7. Not as experienced as some guys on here, admittedly.
I personally love OSX and will always recommend it if I can. However, the problem is that it is not and OS that has been widely proved to be reliable in data centres. Microsoft, for all their faults, has done a *lot* of work on running OSes in the kind of large scale deployments needed for datacentres.
BTW, you may know what's possible with Darwin, but have you ever run Darwin on the number of computers needed to run iCloud?