In my day, you had to be an 'able-bodied' adult to sit in the emergency exit seats, which were generally the ones with extra leg room.
So, when people thrust those letters at me, after I had confirmation that they were not 100% physically fit/able, I considered it would have been negligent for me to allow them to sit in those seats.
For me, that is the crux of it: a medical condition disqualifies you from access to an emergency exit seat.
Some people had already paid extra for those seats, but had to be moved because they were not physically able. I bet nowadays, if that was the reason given, there would be shouts of 'Discrimination!', but it's an operational requirement. I would hate to be on the plane where, with no cabin crew present, someone who could hardly walk was the first in line to attempt to open the doors.
I don't fly any more. Due to hip problems, I have a sitting disability - I can't sit upright in a normal chair, and flying is torture for me as it's not possible to recline seats for take-off or landing. I took a week to recover from my last short-haul flight.
If flying is truly that painful, you can't fly. Full stop. Suck it up, and move on. Holiday somewhere you don't have to fly to.
If you still need to fly and you can't afford an upgrade, there are charities out there that may be able to help subsidise the extra cost. If you have a physical limitation, do some research beforehand of how the airport and flying experience will affect you. Ask for assistance to get to/from the gate (the amount of people I saw who didn't do this was unbelievable). Confirm how much leg room there is on your chosen flight/airline.
However, you do not turn up at the airport or even board a plane, and expect special treatment. Not without researching it, booking it, and (if needed) paying for it.