Depends on the reason for switching. If I'm asked to switch because the new stack is going to have measurable improvements plus the language is going to stay and grow (just as examples may be Go or Rust right now), then sure. On the other hand, if the reason is because the company fucked up by relying on one idiot with an elitist mindset about programming languages, then that request going to have to come wrapped with very good incentives. If I'm a great java developer with tons of jobs to choose from, you are going to have to give me a very good reason to narrow my opportunities down for you. Otherwise, I'd say no and move on and you can go hunting for devs for a niche language and pray to Gods that the hiring situation will improve for you.
Your example of Java to Python doesn't represent the gravity of switching from Java to say Erlang or Haskell. You assume someone says no because they are unwilling or unable. We are unwilling, sometimes, because of the opportunity cost we have to pay for your fuck up.
> If I'm a great java developer with tons of jobs to choose from, you are going to have to give me a very good reason to narrow my opportunities down for you.
Why would learning a new tool narrow rather than widen your opportunities?
Learning an obscure tool that eventually dies is a waste of time. Think learning a language only you and a few of your friends speak. This in itself is good enough reason to be cautious about what you invest your time in learning.
Languages and specially the tool chain and the way of doing things around them evolve. If you spend like 5 years doing something else and come back to it, you'll find things have changed a lot. You may argue that it's easy to relearn this. But from a job perspective, the guy who has been using the exact same tech stack in the recent years will often be a better bet than a guy who wrote Java in version 1. This is very true for C#. Syntax, design principles, tools, method of deployments all have changed rapidly. Professionally, you want to be as up to date as possible in the area you worked in or you get rusty. You can learn new things on your time for their own utility. Flushing down 40-60 hours a week on a useless piece of knowledge is bad all round to me.
> Flushing down 40-60 hours a week on a useless piece of knowledge
That is only if it's actually useless. I haven't programmed in haskell for many years, yet I am very glad I did. It was hard at first, but it made me think about code, data and transformation from a different perspective, expanding the repertoire of paradigms, regardless of which language I code in.
YMMV, but personally, I'd much rather hire or work with someone with 3y java + 1y erlang experience, than just 5y java experience. In general a polyglot rather than monoglot, especially if it's a different coding paradigm. Nevertheless, I may not actually want them coding random bits of work in erlang or whatever random language.
Your example of Java to Python doesn't represent the gravity of switching from Java to say Erlang or Haskell. You assume someone says no because they are unwilling or unable. We are unwilling, sometimes, because of the opportunity cost we have to pay for your fuck up.