Look at it from a Fortune 500 perspective (yes obviously exceptions and I'm not saying I agree or disagree). Do we want to run all or part of our business on an open source tool/project which may or may not have a large community around it and may or may not be around tomorrow or do we use the company who is financially committed to X tool and has virtually unlimited money in the bank?

This doesn't just apply to Microsoft... look at the money Oracle, IBM, SAP, etc. bring in. As the saying goes "No one has been fired for using X software before".

"What happens when (say) .NET isn't the best tool for a particular job?"

So like when you need to write something in Python (http://pytools.codeplex.com/) or Node.js (http://nodejstools.codeplex.com/)? Yea that is all supported in VS by Microsoft. Most people don't realize at some point over the last few years they changed from the MS or screw off attitude to a more let's support what we can one. They have also started opening sourcing a lot of their stuff.

I came from Unix/Linux/PHP and Perl at the time and once I got the hang of C# and Visual Studio I have not looked back.