Runway 31 your point is reasonable (and reasonably made) but sadly, the world isn't that black and white. There are people who can manage a certain distance, but not more, and others who choose not to bring their own equipment for a variety of valid reasons. One example might be some of those who use electric vehicles to get around town (the ones who proceed through Sainsbury's vegetable aisle in a manner reminscent of Boadicea) but do not possess a wchr. There are others who (as excellently detailed by Roghead) have valid personal reasons for wishing to minimise the fuss attendant on their situation.
Once again, I don't believe that an airline should be allowed to cherry pick those members of the human race that it approves of and penalise those that it does not - the world found out why in 1945.
I'm not suggesting that O'Leary et al are explicit Nazis, but I do feel that this would be the top of a very slippery slope indeed. If one of my passengers falls sick with coronary problems in flight, then the crew are required to spend a disproportionate amount of the short flight time attending to him, rather than others. This often means that the company loses revenue since a retail service may not be offered. It also lessens the attention that we are able to provide to others, and could incur significant expense in the event of an offload with delay or diversion. It ramps up the workload of the crew all round, and more particularly that of the Commander if a diversion is required, possibly not under ideal circumstances. If the sick passenger is also a smoker and eats and drinks to excess as part of their chosen lifestyle, then the question arises as to who should pay for all this and also as to whether or not the passenger should be boarded in the first place? A case can be made for applying financial sanctions against anybody who does not fit an airline's own concept of an "ideal" passenger. Let's not go there...