In my experience seniority only guarantees you a shot at a command once you met the command minimum requirements. If you don't meet the minimums you cool your heels until you do.
I believe it was ever thus in most airlines. Basic requirement was senior licence, type rating, seniority, suitability and hours, sometimes a type rating could be a part of the command training. Letting somebody hold a command slot when 4 to 5 hundred hours short is, to me, quite incredible.