The problem with seniority lists , like any military hierachy, is that it also allows completely dysfunctional and personality deficient people ( even though they may be technically qualified) to get into positions of enormous power and authority,( enabling them to make everyone else's life unpleasant), simply because they are in the system
Requirements for a command:- Licence, experience, ability, seniority and last, but not least,
suitability.
So your scenario shouldn't happen and if such a person does make it to command then it is just as likely to happen in an airline without a seniority list as it is in an airline that has one, since management obviously thought they were suitable for the job, whether you like him/her or not.
I've yet to come across an airline where captains were selected by the co-pilots on a popularity basis
