It seems to me that the proponents of seniority as a "sacred cow" not to be touched, are mostly senior types who have much to lose by a system of merit.
The idea that you have a right to command or type conversion is ridiculous. A company invest a great ammount of money into training, why should it therefore not have the ability to select those in whom it wants to invest?
I agree with the concept that seniority, or length of service should be recognised and rewarded. In an ideal environment all should be capable of type transfer or command, and have reached the required experience level when such an opportunity arrises. However we do not operate in an ideal environment. How many of you have sat next to a F/O that will never have what it takes to command or the Captain that no-one can explain how he passed the command conversion. Why should a company have to spend money and training resources on someone with no real prospect of completeing the course. It may be possible in a large company such as BA, where the training budget is huge but not economical in smaller operators.
Are pilots affraid that they might not be up to scratch if a system of merit is used. The contention that advancement will then be at the whim of a management type surely does not wash. This is not a criticism levelled at other areas of industry where this is the norm. Also a manager who does not select on merit, but some cronyist or nepotic way will be found out when they do not perform.