from a USA perspective
sadly, southwest airlines (the model low fare, low cost carrier) charges larger people for two seats and has made that fact well known.
as to denying flight/denying boarding, I have had to do that to a person who requested oxygen prior to flight. pax had a note from a doctor requesting oxygen. the low cost carrier within a carrier had a policy of not providing supplemental medical oxygen, while the higher cost carrier would.
I even got medical advice on the subject, and we offered to rebook the passenger on the other side of our same airline.
as to "WHAT HAS HAPPENED TO AVIATION", it is a sad state of affairs, and of course it is all about MONEY.
funny, in the USA, there was more leg room, faster flying times, better looking flight attendants, and happier passengers 40 years ago than today. EG: flight time between New York, and Miami is longer today than 40 years ago! (planes flew faster then instead of fuel efficient profiles now)
and this is progress?