"Before the usual AC trolls come out calling me a fanboi or whatever, it might be worth pointing out that all the series players - Amazon, IBM, Google, FB, Twitter and so on, they all use Linux / open source solutions."
Actually, although the foundation of what they use is indeed the Linux kernel many of those vendors have put a whole team of programmers to work in order to shape their Linux environment to match their specific needs. That is something many people forget to mention: while those companies may use a Linux solution its not Linux as we commonly know it. Most of them don't simply download a distribution and rely on whatever that manufacturer provides for support.
Which is something most companies do tend to do; they pick up an existing environment where the goal is to get to the result as optimal (or as easy) as possible. Once a product isn't supported any longer they usually move on to the next supported version.
And this is automatically also an argument as to why Linux isn't the best solution by definition.
When looking at such environments: Windows Server 2003 was released around 2006 and support stops around 2015. That's 9 years worth of (continuous) support. You can see Microsoft's own comment on that here.
Around that time (2005) Debian 3.1 'Sarge' was released. Its security updates stopped around 2008, that's merely 3 years. Read about that here.
Sometimes one needs the robustness of Linux, at other times the extensive support of Microsoft is in favour. That's the way the real world works.