Some pilots and I were discussing the reason for seniority. It's common sense that given the opportunity, lackeyism will take root in organizations. While the seniority system sometimes results in the "unworthy" being promoted, at least it has some fairness to its design. The promotion of friends and butt-kissers that would inevitably come to a large organization (like an airline) without a definitive system like seniority would neither promote skilled individuals nor promote company loyalty. It may sometimes get it right, but as seen in politics it would eventually devolve into "my buddy will be your captain" no matter how inept my buddy is.
It was just taking the better of the two options.