Of course people can learn new languages, but:

- A nonzero number of excellent potential applicants aren't going to work for your company because they don't want to invest their own development into learning the Clojure ecosystem deeply. (This might be somewhat balanced out by folks who are stoked to learn Clojure.)

- Every non-Clojure person you hire will require significantly more ramp-up time. This can be a real problem for small companies where you lose quite a lot of expertise every time a senior developer leaves.

- These problems compound for every "innovative" technology you add.

Learning a language is a piece of cake, learning the ecosystem is another matter.

Of course they can learn new languages. But it takes some time to become really proficient in a language. The question is how many languages do you want to support in your company? Because it's not realistic to think that developers will not need to move from project to project. And in fact when things get busy they may need to swap from one project to another and back on the same day. It's not ideal but that's life. The inefficiency of swapping between languages will often outweigh the efficiency of the new language.

Google were notoriously strict for limiting languages at the start and I think it was very frustrating for some of the developers they hired. If I had a boring technology checklist then "Language we already use" would be top of the list.

If it’s a language I’m not familiar with, I won’t consider applying, so that’s where the story ends in my case. I could be an outlier though…?

The majority of companies want you to be productive on day one.