Seniority systems have been around a very long time and when they became the 'norm' the vast majority of pilots were happy to stay where they were, a few dissatisfied individuals might move, the penalty was losing seniority.
As has been stated many times already, seniority only determines the order in which you get considered for command, you still have to meet the suitability, experience, competence and licence requirements, not to mention passing a command course.
The people that have the biggest problem with seniority lists are those that think they are better than they usually are or have left one company to join another and find they have less experienced people ahead of them.
Back to paragraph two, "seniority only determines the order etc. etc."
The only seriously iniquitous use of a seniority list I ever came across was in Gulf Air in the seventies. A bright young spark on the PLC convinced the council they should have a 'D' list of FOs who had an ATPL from which future commanders would be selected. The iniquitous part came when it was decreed that when one did get their ATPL they moved on to the 'D' list at the bottom and not on DOJ! Needless to say, 'bright young spark' joined with an ATPL and didn't want to wait his turn, (nor did he!). Looking back over the years it probably didn't hurt me that much but I definitely didn't like it at the time, DOJ was GOD!.