I can teach any developer Rust or Clojure in a couple weeks. I've only done a few hours of Rust study myself, and just minutes in Clojure and yet I will say that with confidence. It takes years to master, but the difference between 1 month and 10 years for an otherwise experienced programmer is not very large - either way the hard part is the problem domain, understanding your code, and other such details not related to the language. HR puts far too high a weight on skills in a programming language, in part because they are easy to measure while the ability to write good code is hard to measure.
The longer I am around the less true I find this. Can any good developer learn and contribute with a new language? Yes.
But there is also a peak performance achieved by using tools for decades that is not transferable. C experts didn’t switch to Java, they found different jobs.
For your core technology you want a few of those 5-10+ year guys.
