That still takes time. If someone is already experienced in an (complex) ecosystem, he or she will be way more effective in it, than someone who has just general skills. I guess I am a very good problem solver. But if I suddenly would have to do a C++ project with all its footguns, I am not aware of (last time I coded in C++ was in university)? I would suck in it for quite a while. This would not be effective at all, unless I would want to change my skillset to the C++ ecosystem.
Yes. I want to work somewhere that shows they're committed to the long term, not somewhere that's focused on how much I'll get done in my first week.
I'm not sure. I'd guess very few companies that hire more than 20 devs a year would have any language requirements. But I don't know what % of jobs are at companies of that size.