JazzyK,
Are you suggesting that the union/association that represents all the pilots in BMI goes to the new employer and says "sorry but we would prefer you keep pilots 1, 2 and 3 and make pilots 4, 5 and 6 redundant instead because pilots 1, 2 and 3 have been with the company longer". If I was pilot 4, 5 or 6 I would be most upset. to say the least....
Many people have very valid reasons for having a lower seniority than other colleagues e.g. sometimes they weren't born when someone else joined the company or perhaps they spent many years of their life serving their country rather than going straight into a much higher paying civilian job. Why should they be any more liable for redundancy than anyone else?
As has been said already you cannot sack an individual from a position and then place another individual in that same position.