> Why isn't the plan here to systematically start improving DOM performance,
There are already many steps taken to improve DOM performance over these years. However, DOM is designed for documents. The performance can never be good enough when it is abused for non-document usage.
> or create APIs to more directly modify how elements are laid out and created? Why do all of this work to benefit only Google Docs?
Because other browser engines are unlikely to adopt these APIs just so that Google Docs can have better performance. Not to mention that these new APIs will take years to be present in every user's devices.
There are already many steps taken to improve DOM performance over these years. However, DOM is designed for documents. The performance can never be good enough when it is abused for non-document usage.
> or create APIs to more directly modify how elements are laid out and created? Why do all of this work to benefit only Google Docs?
Because other browser engines are unlikely to adopt these APIs just so that Google Docs can have better performance. Not to mention that these new APIs will take years to be present in every user's devices.