Companies like Apple actually take accessibility really seriously though, which I applaud. That said, there's a pecking order to disabilities that these companies have to service. I can tell you that blindness is top of the heap and cognitive disabilities are bottom of the heap. It's a crummy situation for users like you when a disability doesn't weigh as heavily as the product design goals - grumble, the world is broken.

I imagine a headphone jack must use a lot of space within the case, which creates a lot of hardware design headaches that go beyond waterproofing.

I see some people at my workplace who walk around all day with a full bluetooth headset around their neck (not earbuds, like the big cans type) even when not in use, almost as a fashion accessory or statement. Perhaps you could experiment with those kinds of bluetooth headsets if you upgrade to a new iPhone. Obviously that doesn't solve your headset needs for running though, grumble grumble.