Progression through the ranks in a major airline is based upon seniority because all pilots have achieved the same high standards and qualifications. Otherwise they would not make the grade. Seniority is also another word for commitment - it is also a measure of loyalty to that particular employer. The more proven loyalty gets rewarded. Whats wrong with that?
If progression was to be based upon any other criteria, it would, as often does, lend itself to promotion and advancement on the basis of politics. We are all well aware of those airlines where direct entry from the military guarantees instant promotion ahead of all others.