Seniority is one of the main commandments of unions and unions are what shaped the current system. You should get a copy of Flying the Line to get some significant background on the history of unions (ALPA in this book) at the airlines. Some people like a union and some don't, but if you read this book or go to work for some non union companies as a pilot, you will probably soon realize the necessity of unions for our profession. There are some places that is has worked out well without a union, but those are few and far between in aviation.
I believe American Airlines is were the B scales and first year at hideously low pay started and the other airlines went with it as soon as they could.