Your expensive vendors sit on security patches for weeks and my free ones shoot them out almost immediately. Why is that not a business concern?
I don't think it's zelotry, there are a lot of advantages to open source software you aren't going to get from Microsoft. If you are an edge case and you happen to stumble on that race condition bug are you going to have your engineers black box test and reverse engineer someone elses product illegally while they wait around for Microsoft support or have them look under the hood, patch the bug and move on?
What the big licenses fees get you is accountability, which is of course a huge thing, but what open source gives you is control.
I think if your business is software, then open source makes perfect business sense, especially if one of your assets is a team of competent engineers.