Developers do need to pick a set of core technologies, and stick to it or a career can fall apart I bet. I don't know from experience. I've been a Java DEV ever since MSFT tried to hijack the Java language with a proprietary version back in 1998, and I abandoned MSFT and never went back. I can imagine trying to be both iOS and Android developer would be about as insane as trying to be both .NET and Java developer. Oi…
Sure it does. We do consulting projects in Java, .NET and C++.
Right now I am doing exactly three customers in .NET and one in Java.