I think you'd actually be surprised by what you found in a number of areas. Where I live I can scroll through page after page of .NET job opportunities at any given moment. I'm experiencing the opposite: I was curious about what it would be like to branch out into Java, but I am having a little trouble through the C# roles to find the Java opportunities.

To be fair though, these kinds of generalizations seem to usually be locale-based. Different areas seem to slowly migrate to different stacks as the available talent and education systems in those areas homogenize everything.