The vote to bring in PP34 and put all new pilots onto it was voted through to keep PP24 and to prevent bmi being set up as a lower cost subsidiary at LHR.
Simply not true. With the advent of the age discrimination legislation increasing the retirement age by 10 years BA would not countenance paying a PP24 salary to a pilot for an extra 10 years.
If I recall correctly PP22 - 24 represented a significant jump in salary when compared to earlier incremental rises. This came about to benefit the pilot's defined benefit pension which was based on the salary earned in the last few years.
I think the theory behind the PP34 scales was that individuals would only spend a relatively short period on the top scale, as was the case with PP24 and retirement at 55. Clearly someone already on PP24 when the retirement age changed hit something of a jackpot.
The introduction of PP34 scales took place at the time of other negotiations but the basics above are what actually drove the change.
It was ever thus that there is always a large amount of luck/bad luck involved in aviation careers.