One thing that is certain in all aspects of human life is that there is no "one size fits all" situation that is ever best for every situation. Seniority is one of those exact situations. It may suit the likes of BA but it most certainly does not suit everyone.
So then it is perfect as it is right now isn’t it?
Some airlines use seniority and some don’t. So everyone can choose what suits him or her best.. Problem solved!