You often have to learn this stuff outside of your regular job. Once you know another stack then you can find another job using it. That's how you exert control over your career. Tying yourself to one company (or stack) is a slow death sentence.

EDIT: to those downvoting this... Do you disagree with me (i.e. you think it's a good career decision to stick with a single tech stack) or do you agree but dislike that it has to be this way? Genuinely curious because I didn't think what I said was controversial at all, just industry observation/common sense.

I live and work outside the valley, and I know python and Java, dabbled in scala, rust and lately C# -- no one out here will give me a job on any of those languages because I have no professional experience with them, and some of them no one here is using yet

You're being downvoted because you are arguing the details and have missed the wider point being made.