> 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.
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.