While I mostly agree with your post, you went a bit overboard.
These operating systems can make GREAT servers. Linux definitely has a place in the business world. And yes, even OS X does as well. For some reason, Photoshop and "creative" apps run better on a Mac. I defer to those people, since I'm not "creative." :)
I have worked for 3 different companies in an IT Administration capacity during my professional career (10 years). I LOVE Linux. I use it at home, and have tried using it at work as a workstation. I have my own NetApp filer that uses a Unix derivative (ONTAP) to publish files shares and authenticate against AD. At work, Linux interoperability with Windows is decent, but I am more productive if I use an actual Windows machine to administer a Windows environment.
I would never push Linux or even OSX as a desktop replacement in our medium size 1000 user environment for multiple reasons. The biggest one of those reasons is a lack of centralized management that is as effective as Active Directory. Having AD in an environment and being able to control it via central management is HUGE when you are management and you need to have a solution that can administer your users and machines in a cost effective manner. Additionally, you will need to retrain your staff or hire replacements who would subsequently demand more salary.
Or, you can just stick with the known evils you have and do your best to overcome them.