Sure, but if you have to pay a premium for regular seats just because you have a disability, that seems like a problem. And the injury scenario could also be extremely problematic. What if there are no sitting seats left, or you could afford the cattle class seat but not the business class seat?

If my grandmother needs to fly, do we have to upgrade both her seat and someone else’s in the family to something twice as expensive so someone can help her with the various things she can’t do for herself any more? Or upgrade the whole family if we want to sit together?

Current cattle class seats aren’t comfortable. But they’re accommodating to the elderly and people with disabilities or injuries. If some seat shuffling is required to accommodate a special case it’s usually just shifting people between aisle and non-aisle seats, not downgrading them to standing seats so someone else can sit.