Deep expertise isn't as useful day to day as we often portray. With a little elbow grease one can get to a place of being more than baseline productive in relatively short order. Depending on the language I expect a reasonably good dev to be able to be "fluent" in the range of a few weeks to a few months. And it's not like they'd be useless the entire time before that happens.

To me, that's a reasonable investment to make.

Edit: This does require a certain persona in the space of "reasonably good dev". People who have spent their entire life focusing on a single language or paradigm are a lot less likely to be able to shift gears. People who have broad exposure to different concepts are more able to say "How do I do X in Y?" and stop fighting their new language.