I don’t buy it. As software developers we learn technologies for breakfast. We learn them, and then we forget them. We recollect the wisdom gained and apply it in our next gig using a different technology. It works for mobile development, for frontend, for backend, for infra, for almost anything. I have worked for years with PHP, Apache, Ansible and MySQL. Nowadays not anymore. And in 7 years I’ll be using something…
Yes because I know Java (hypothetically I haven’t touched Java since two years after it was introduced) I can become an efficient Android developer “in a few days”. Better yet, since I did Windows CE development for ruggedized devices using C# Compact framework back between 2007-2010, I should have no problem picking up iOS development “in a few days” and start being a productive member of a team right? Or better yet…
I've long said, and would stand by: Programming/Development is logic; learn logic. Languages are just syntax; if you understand logic, you _can_ learn the syntax.
Taking concepts from one language to another shouldn't be an onerous task.
Of course, none of that means you'll get that _senior_ language -focused position, or any particular position at all.
But let's not act like picking up a new language, for a developer, is the equivalent of switching from development to medical practices, or even networking/system administration/etc.