> The first can be remediated by planning to bring new hires up to speed, and just making that investment in them. (It can also be a useful filter

It can be a useful filter in both directions. I'm more impressed with prospective employers who have an onboarding plan including some reasonable model of a learning curve than prospective employers who assume "oh, we use popular language/framework/tooling x/y/z, so if we find someone who knows these specific things, onboarding will be a snap or practically take care of itself."