Actually if you have any old timetables you will be surprised how infrequent flights to long-haul cities really were. This extra capacity is there to serve the vast number of people who fly. 20 years ago only 20% of US citizens had ever flown – now its 85%.
The real thing is convenience to Australia: 2 stops with shorter transfers and better journey times are available from anywhere outside of London without going via LHR. Add the extra cost of sending UK based aircraft too Australia and I completely agree – the only thing keeping this going is cargo or enough people who want to fly BA specifically despite the extra inconvenience for many. A niche route at best
What this has to do with quality of seating im not sure though.