Don't forget those salaries are exclusive of bond repayments - £12,000 per annum and flying pay which should be worth at least £7-8k. That puts year 1 total remuneration back towards the low £40k mark, which personally I think is quite respectable.
I reckon that the number of FPP cadets who 'use' this scheme and intend to leave is pretty much zero. The selection process itself should have weeded those ones out.
BA seems to be able to cater to a lot of different career aspirations in terms of their bidding system. With enough seniority, if you want day trips and home every night, you could do it. If you wanted longer trips away or long haul, you can do it. There are plenty of SFO's who have enough seniority to bid for command should they so choose, but they prefer to wait in the RHS because of the lifestyle and roster freedom that is available to them.
It's also worth pointing out that if you have aspirations to do other things in BA than being a line pilot, there are plenty positions available to you that don't require a command as a prerequisite.