Regarding the last paragraph, I think an experienced developer can jump to new frameworks relatively quickly. They won’t be putting out PRs on day one, but can certainly do so in a week or two with a new framework. If your web fundamentals are strong, one can get through the docs for a new framework in a few days, then spend a couple of days looking at GitHub repos that use the framework in question to suss out best practice patterns of code organisation and you’d be in a good position.

It’s odd to me that frontend dev jobs ask for experience with specific frameworks, since most of the popular ones share core fundamentals a lot of the times. It’s no different than going to a new backend role to work on a large existing codebase. The codebase will have its own homegrown patterns and concepts to get familiar with anyway, how is this any different?