[simonh]

I’ve known people who built lucrative careers on expertise in specialised technologies, and others who’s entire careers disappeared because they were too specialised and the technology got deprecated. You might be fine, but I’d definitely recommend diversifying your skill set a bit. Being a generalist has served me well.

[SZJX]

I would say every capable programmer should be a "generalist" in a sense and be able to transfer their skills with relatively little effort. If they can't, they are not really that good at the abstract concepts. Still this doesn't prevent one from focusing more and specializing more on one field/language that they love and be as familiar with it as possible.