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". A lot of companies are looking for that expertise now, not to spend 6 months training you on the job, especially at a big salary and title.
The problem I encountered as I hit the 10-year mark is that it is hard to get the interview when you have extensive professional experience and none of it is in $TECH the company is looking for.
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.
> 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.
I haven’t found any of them difficult to learn though, I code in all three along with Python, almost daily. I think it’s mostly just doing - I get the books out all the time.
Decades of experience in C and Java ought to work just fine for someone who wants to try something new, like React. I learned React after decades of experience in other things, and I loved it. I don't see why it should matter what I did in the past. Was I successful? Did I enjoy it? Will I come to work every day energized and ready to work on new things? Those are the things that matter.