Why do they need to open the OS up anymore than they have? Their accessibility APIs are already industry leading, constantly being worked on and provides the ability for devs to make heaps of apps on top of it.
What more could you want? I like that apps can extend the usability of macOS in ways that Apple wouldn’t do themselves. And if I don’t like a specific app, I can just remove it.
Genuinely curious would opening the OS would do.