The simple credo 'I f they can come the airport wihtout a wheelchair then they can go to the aircraft without one ' isn't much of an argument in today's airports.
Those of here us that fly regularly witness plenty of pax that have difficulty with walking large distances and particularly steps. Throw in the stress of check-in, security, a never ending walk to a cramped flight, sit in a dehydrated atmosphere and get spat out the other end and for an amazingly small number of paople it is a little much.
I keep reading here about the low cost model. As FR have found out it is the law to look after your pax and if to achieve quick turnaround times you try to discriminate, you will be punished.
BTW on the low cost stuff FR charged stranded Jetmagic pax €220 one way EICK-EGSS.