6-String: Your perception of the seniority system is undoubtedly sincerely held - but it is sincerely wrong. I have worked for 5 airlines - only one of which had a seniority system. My current airline, easyJet, does not have a senirity list but I hope that one day it will do so. You rightly point out that seniority is not practiced in many other industries but you fail to mention the unusual nature of airline operations. People in large national carriers (BA lin your case perhaps?) have to wait many years to be eligible for promotion and in doing so build up vast amounts of experience. The overwhelming majority of them are then ready for promotion but there are plenty of ways to stop 'unsuitable' candidates from being promoted.
You are making a veiled reference to 'imported experience'. My own experience of such views is that the only people who hold them are people who feel they are 'better' than the people above them and just want to jump their turn. If someone has demonstrated loyalty to a particular company he should, quite rightly, receive first bite of the cherry when the good deals come.
You suggest that 'marginal' individuals are promoted in a senority-based system. A proper seniority system has protections against that - minimum simulator standards, line checks, personal debriefs and so on. There is ample scope to prevent the 'wrong' people being promoted.
If you rid yourself of the seniority system then the only valid replacement is 'merit' - which in practical terms means corruption. The real problem is who decides the merit order. Before you know it you are having to have the 'right' people round for dinner, be seen in the 'right' places, say the 'right' thing to bosses etc. I have worked in such environments and frankly despise them. Seniority is the only fair system - it works and will continue to do so for many years to come.