During pension negotiations the BACC told us (the members) that we could not strike over the issue of new joiners being excluded from the FSS until BA recruited a pilot on that scheme. This happened two years later. So a ballot of 3,500 pilots asking will you go on strike to force BA to put new joiners on the closed FSS would have been a foregone conclusion? The scheme was closed to all other new BA employees too so another can of worms there.
At the time, there was a great deal of emotion surrounding this issue, with many saying that they would strike rather than create a pseudo-B-scale with inferior Ts & Cs - exactly what the pension issue has resulted in.
Regardless of your exact opinion on the outcome of that issue, and its effect on the long-term, it point to exactly the reason why modern unions are useless in workforce protection - it is only ever useful to aid the workforce in place at that time in an employer, and of no use whatsoever to those who have yet to join a workforce. A union contributes nothing to expansion of employment and is not incentivised to do so.
While you argue that it is the choice of those new recruits to join, and the conditions are clear, that is not relevant. The workforce at the time made no effort to prevent the B-scale arising, thereby diminishing the size of the pie for those joining in the future, for their benefit alone. To follow the argument through to its logical conclusion, one could say that since Victorian mills' working conditions were known to all and people still sought employment there, why should conditions change? That is contrary to the foundations of unionisation in the first place, but is the core of what you suggested regarding the pension issue.
BALPA does have a place - it should be involved in safety issues, supporting those in legal wrangles with their employers, and preventing the company from infringing on the ability of the commander to undertake his duties.
As I have argued many times in the past, the seniority system is to the detriment of the employee, prevents mobility, and depresses employee wages in the long-term. Younger employees, who will not have their turn, who will not attain the same pay scales where they have had to move employer through necessity, who work on below-market pay points, and who do not benefit from DB schemes - those people will one day refuse to support the elder employees when management turns its focus on them, by virtue of their inaction in the past.