I'm not sure that's unreasonable in general and it goes beyond just $TECH. I have experience and skills in a lot of different areas and, sure, I could pick up new skills in related areas of business and tech fairly quickly. However, if a company is looking for a senior person to "hit the ground running" in an area where I have at best peripheral experience, I wouldn't seriously expect to get hired--especially as an IC.

The details matter of course. If $TECH is something relatively new without a lot of experienced practitioners, it may be reasonable to hire someone with a lot of experience in somewhat related techs. But no matter how skilled the person is, you're probably not going to hire someone for front-end or mobile development who has only ever worked on back-end infrastructure.

Right, it's not so much a reflection of age-ism as it is just not having the skills in the job description. It doesn't matter if you're 25 or 45 - if they need X years of language Y, it's for a reason and 20 years of generic dev experience is not usually an acceptable substitute.

I don't understand the point you were making in the original post then.

> What you don't want is to interview for a React job with only a cursory/tutorial knowledge of it, and try to fall back to "well I'm an expert at 10 other languages, it's all easy"

Of course I would expect someone who claimed professional experience with React to have more than "a cursory/tutorial knowledge of it". In the post that started this chain, though, you said

> Decades of experience in C/Java is not going to work at a React shop. Either learn (and be opinionated about!) frameworks built in the last 5 years, or stick to somewhere within your expertise

I take that (in combination with the above) to mean that you somehow need to get a job where you can use React in order to get a job where you use React. I'm not sure what the point of that advice is, then. Perhaps I'm misunderstanding something here.