I think a well-rounded developer should appreciate the strengths and weaknesses of different languages, but I wouldn't need or expect them to know all of them. I really think displaying well-rounded fascination is valuable, but you can develop an expertise.
I think the tee shaped skillset is a great model:
I also think it doesn't matter which area you choose to go deep in, because in going deep in anything you're going to incidentally learn a lot about other things. If you allow yourself to seriously go deep on a subject with purpose and a healthy amount of obsession, you are going to learn a lot about a lot of things. And the things you don't know, you'll have a lot more confidence in your capacity to develop an expertise that you could jump into that stuff later.
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