Shallow gene pool - done plenty of duties approaching 14 hours lately with the pax at the end. Admittedly not 12 hours, but the bulk of my duties are towards the 11 hour limit.
Belowmda - although it sounds like it's worse in the FO rank plenty of guys aren't taking up Bus commands, so their first command is on the Boeing that they've sat in their whole career.
Personally I won't leave the Bus because I make more money here. That said I'm knackered and filing plenty of fatigue reports and taking sick leave because I'm run down and getting sick.
It might be shocking to some but not everyone LOVES being on a plane. I'm not passionate about it, but I'm well qualified and I do a good job, but it is only a job. I don't get any more satisfaction out of being a Bus FO than being an SO. For me it's about money so I'm on the Bus. For others quality of life is more important so they stay as SOs. Anyone who doesn't understand that probably isn't in ANZ and therefore doesn't understand our progression/seniority system/bidding etc.