It is very easy to understand why it was voted in - BALPA recommended it. Within BA, pretty much all ballots go the way BALPA recommends. A very large proportion of the membership cannot even be bothered to read the detail of what they are voting for.
I agree with your point about the sustainability of rosters at the bottom of JSS. BALPA are desperately tweaking inhibitors in an attempt to find a solution which is not as drastic as rolling seniority, but it has not really worked so far. I think the time will come where they have to at least vary the block of seniority affected by global constraints and crew repair. Hitting the same people over and over and over with extra work is not going to be tolerated in the long term. In the past, extra work was given to those who were not working so hard. With JSS, extra work is going to those at the bottom until they can work no harder, regardless of how hard the senior guys are working. That is inefficient rostering and will lead to sickness at the bottom, which will lead to more inefficiency. BA will not accept that and will demand solutions.