I can only talk about my own motivation for repeatedly bringing this up. If a developer is unaware that they're choosing an inaccessible GUI toolkit for their application, and that application is then required for a particular job, then that developer may end up unwittingly preventing some people from doing that job. And this isn't just hypothetical for me; I know of a blind person who lost his job (luckily only temporarily) because of an inaccessible application. So I think it's entirely reasonable for an application developer to dismiss a GUI toolkit because it's inaccessible. I wish more developers would research this on their own before choosing a toolkit for their applications. But since many don't, I feel obligated to draw attention to it. I'm glad I'm not the only one.
As for your suggestion that applications should implement the platform accessibility APIs themselves: First, it's impractical to expect application-level code to do this directly. The Windows accessibility APIs are notoriously hard to implement correctly. And my understanding is that the Mac, iOS, and Android accessibility APIs are only easy to implement if the application is written in the platform's native programming language. Now, these problems could be solved with an open-source wrapper library -- the SDL or GLFW of accessibility. And I'm planning to implement such a library myself sometime. But my plan is for that library to be used by toolkits, not applications. Because if every application has to directly implement accessibility in parallel with its GUI, then it's a safe bet that even fewer applications will be accessible. We want to get to a place where as many applications as possible can be accessible by default, with minimal effort on the part of the application developers.
A graphics application should be able to submit draw calls or queues to a OS subsystem that, for example, generates a comparable audio space, performs colorspace/size transformations for improving visibility, etc. Developers should just be responsible for plugging in to these systems, and the systems themselves should be as standardized as OpenGL, Vulkan, DirectX.
I don't know if there is a "Khronos Group" for Accessibility Standards. Maybe there is.
I suspect most Devs would never link up and keep up to date an alternative accessibility interface. Also there is a lot of benefit to the accessable and standard interface being close, to make it easy to cooperate.
My opinion for the last several years has been, if you use these toolkits, you might just have to accept that your app is not going to be accessible and is not really suitable for anything beyond highly specialized use.
It's not at all clear to me that the right approach is to have both the accessibility interface and the GUI be generated from the same components. It seems to me that it's just approaching the lowest common denominator of a passable GUI and a passable accessibility interface.
The traditional widget-style GUI paradigm asks you to essentially copy all your application data into their format and to keep this format in sync with your own data. This is tedious, and I believe hinders the creation of better GUIs. On the flip-side, owning your data structure means it can indeed hook into the accessibility API for you.
Immediate mode GUIs provide an alternative approach, where they only handle rendering stuff on screen. Since all the data structures are now properly controlled by the programmer, it is much easier to create dynamic GUIs. Taking a step back like this provides an opportunity to further the state of the art.
The solution I see to providing accessibility in immediate mode GUIs is that the programmer has to hook into the accessibility API themselves -- if it makes sense. It's not clear to me that certain advanced GUI programs can ever be very accessible. Yes, this is more work, but there's also an opportunity to create richer experiences for people with disabilities.