Without seniority system,pilot's carrier would becomes a 'dog eat dog' world.It is
going to be harder when one grow older.If you can rise faster as an FO then you will fall faster when you are a captain.
With seniority system,promotion path is in a more orderly manner.If the airline is expanding,you promotion is faster.But if the airline is matured,promotion will be slower,you have to jump ship for a faster carrier path.If you have been sitting around waiting years for your command and thinking that seniority is the obstacle of your promotion path,then SHAME ON YOU! You idiot counldn't count!
Simple calculation would pinpoint where you would be by looking at the company fleet plan.Now stop blaming the seniors and pull out that calculator!