Let’s set aside captioning and focus just on iOS for a moment. My answer is simple: An ignore-tremors mode.
I don’t see how we’re ever going to make a touchscreen accessible to quadriplegics, but if we can install a parallel set of user-interface gestures for blind users, we can install a third set that ignores erratic or repeated motions. Unite these gestures with voice commands and certain existing VoiceOver simplifications (swipe right or left to move from icon to icon, for example) and suddenly people with CP or Parkinson’s can use an iPhone.
If one person at Apple wants this to happen, it will. It’ll never happen for Android or Windows – irrespective of how many people want it, let alone need it.