As an outsider looking in it seems that those who don't have it, seniority in BA, don't want it because it blocks potential career progression in a one step forward 2 steps back sort of way, and negates their bmi seniority which makes the time put in seem suddenly not worth very much, frustrating.
Those that have it obviously see its potential loss as a loss to their perhaps not so quick but I presume predictable career progression, and the associated benefits, and would dilute their time put in to less than its present worth.
Regardless if change is introduced it will be change for the worse I suspect, as the company (as all companies these days) will use the opportunity for cost savings.
My advice for what its worth is when weighing up the pros and cons, is to ask, what are the advantages/disadvantages to me in the next 1-5 years and what are the advantages/disadvantages to me in the next 10-20 years, then take a balanced view.
Reminds me a bit of the retirement age debate.