If it's vitally important to keep up with new technologies in order to remain employable why are the 7 most employable programming languages (C,C++,Java,PHP,JS,Ruby,Python) at least 25 years old? The job market says the opposite - that new programming languages hardly get a look-in.
Programming languages alone aren't the only things you have to know in order to be a professional developer. It's also important to keep up with the ecosystem around that language, and industry-wide trends in application design, testing and deployment/ops.
Good luck getting a Javascript job if all you know is jQuery, and you've never touched Webpack/React/whatever else JS devs need these days. Maybe JS is an extreme case, but some version of that same phenomenon exists for all the languages you mentioned.
