Frameworks aren’t inaccessible because they’re frameworks
First, I agree that the spreading of new JS frameworks is a temporary problem for accessibility, since many developers who build these frameworks do indeed not know, or take into account, web standards such as the correct HTML semantics, or where these do not work, standards like WAI-ARIA. But if they did, the output those frameworks generated would be just as accessible as plain HTML typed into a plain text editor.
I strongly believe that this problem can be solved. The solution to this comes in three main parts.
Web standards and their accessibility implications must become part of any and all university curricula that teach web development. A theme that resurfaces at every conversation I have with hiring managers is that engineers fresh from university have never even heard of accessibility.
And guess who those people should be that teach the younger generation these basics? Right, it should be us seasoned accessibility advocates who worked in this field for 15 or 20 years and know this stuff. We must push for many more solutions to this teaching problem than we have in the past.
Fix up frameworks and teach others by doing so
The second angle to attack this problem from is by fixing existing frameworks and pushing those fixes upstream via pull requests or whatever means is used in that particular project’s environment. And be vocal about it. Explain why you’re doing certain things and what the accessibility and usability benefits are. I know a lot of good people who already do this, and successfully, and have helped to make the world a lot better again for many who rely on assistive technologies while accessing web content. And also include those who benefit, but don’t use such assistive technologies.
And even if you’re someone who doesn’t have the coding skills to fix up the frameworks yourself, open pull requests. Describe the issues, maybe link to some resources about web accessibility. And tell others you opened that pull request, so they may become aware and jump in to provide their skills.
But first and foremost, be friendly when you do those things. The framework authors or community aren’t your enemy. They may lack some knowledge, but that is probably just a matter of improper education at their university (see above), or lack of awareness. Both can be changed, and this is only achieved by being friendly, empathic, and not by putting them in a defensive position by being rude.
Push on implementors to improve standards support
A third point that is equally important is to push implementors, in an equally friendly tone, to agree on technical issues in standards bodies and implement better support for new widgets, improve support in existing ones such as styling for form elements, for example, and thus give developers less reason to reinvent the wheel with every framework. The better the browser tooling is, the more accessible the web becomes from this angle, too.