Here's what I've seen, people getting caught up in learning the "flavor of the week" language and becoming really good at it. But, then, it goes out of favor and they're stuck with a lot of outdated knowledge. It can be difficult to get away from those languages when looking for a job as well since potential employers think that's all you know and won't consider you and those who will hire you need you to maintain an ailing legacy system.
I do think having a strong foundation in a C based language (C++, Java, C#, etc.) is something that will hold up over time vs. more proprietary languages and frameworks. One of the best decisions I made early on in my career was learning C. I doubt I would still be programming today if I had chosen a different language to learn.
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