Seniority is a great idea if you are living in the 60's or 70's. Today it is the biggest handbrake on our lives. Why shouldn't you be able to change company and go to a position that is suitable for your level of experience?
Just because your airline goes broke or lays you off because of economic circumstances or inept management, why should you not have the opportunity to apply for a command at another airline rather than starting all over again?
Seniority is insane and serves only to stop Pilots marketing their skills more effectively (which airlines are grateful for so wages are kept down).