Alternately... the chap in crutches with his leg encased in plaster hobbling down the packed carriage could have broken it (the leg, not the carriage) while skiing, in which case, it's his own fault. He chose to go skiing. Why should I sacrifice my comfort for him?
Indeed. There is no way of knowing whether someone on crutches can be considered to have been the cause of their own misfortune, and why should it matter anyway?
Often we can't see or know what health problems an individual is suffering from, so whether something can be judged to be self inflicted is irrelevant anyway.
All we can do is to decide for ourselves whether we are in a position to give up our seat for someone who may need it more. I am more than happy to do that for a pregnant women, as I am for someone with obvious mobility issues, particularly if I have a seat near the door which would be easier for soemone to reach than having to struggle down the carriage tp a spare seat in the middle!
I make no judgement on others who don't move, as I don't know their individual circumstances. I do what I feel is right in the circumstances. I would assume that in most cases, those who are able to reserve seats will gladly do so, and I don't feel that being unable to book aseat should require someone who clearly has difficulty in standing to do so if I am able to do so instead.
Others can make their own choices!