I don't think
disabled form elements fit in the same category as a potentially
inert element. Form elements, make sense for users to know about because there may be some reason a form element is disabled and can eventually be "un-disabled" by a user interaction for instance. Hence, why form elements should be detected and announced by AT.
inert elements, on the other hand, are elements that would never be helpful for a user to know they exist in the DOM. I would go as far to say that AT shouldn't even detect
inerted elements because, theoretically, they should be in another "sandbox" (for lack of a better term) from the non-
inerted elements in the DOM tree.
I favor an API that treats
inert differently from
disabled elements and
hidden elements. I agree that the
hidden attribute has too many weird implementation details, so I wouldn't encourage extending/modifying it for this approach at this point.