I was an expert at VB6 back in the late 90's before .NET arrived. I knew almost all of the nuances of that language. What good does that do me now? Well, some, since I have to work with legacy code from time to time but I wouldn't want to have a job where that's all I do (and I've been offered some like that).
The problem is that technology changes and it is changing even quicker today. It makes it impractical to learn everything because you will find yourself falling behind the curve. Core language elements and well known nuances and quirks should be your focus. Language trivia is usually only good for job interviews.
What is important is knowing how to find the answer to an obscure language question. Usually you can find an answer on Stack Overflow or GitHub and, if those fail, there are a lot of other sites to look at.
Obviously probably you are in this field even before I was born, so the experience might be telling you something else.
Do you learn/read a book, learn a language that will help you 28 years later using same rules? You don't.
Surely the knowledge you picked up helped you in lot of cases when the new syntax arrived, people asked you to help them rebuild the old stuff using the new shiny system.
One good example would be Crypto bubble, it was short lived, few developers actually got inside of it. However who had deep knowledge in other languages like (C, C++ or even JS) had easiest time absorbing solidity. Whoever didn't spend much time on it had a harder time.
I think the target matters, if one spends time on one specific language and studies bits and part of that to the core, then he will have such knowledge that will help learn new things faster or do unique stuff even better.
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 But, then, it goes out of favor and they're stuck with a lot of outdated knowledge.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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