It's always a good idea to have knowledge of all sides of an application to a certain degree. Of course, the "full-stack" developer is a good fit to modernize or fix any legacy application.
But as you already mentioned, nowadays it might be better to focus on a particular field. Looking beyond the horizon is a good idea, but again, just to a certain degree, I think. There are already so many technologies that want to distract you, better dive deep into the technology you like most.
And regarding your preferred language going out of style: This can happen to any technology. As a software developer, you should always have the skills to learn another language/framework/whatever. Again, let's have a look at web development. So many different languages and tools out there. But if you write your code in C# or Java (just an example!) should not matter. Switching technologies might take you some weeks. But then you know how it works.
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