I approached my boss just over a year ago to inform him I was interested in joining management(there was zero openings at the time), since then my responsibilities have grown immensely, to the point where I'm basically covering my direct supervisors position, along with my own so that he can assist them on another shift. My boss plans on retiring this year and I already know that if I want his position(I've been debating my want for it since a few months after I let them know I was interested) it's mine to have. Had I not made it known that I was interested, it would not be mine just based on seniority.
Seniority should have absolutely zero to do with a job promotion unless all other things are equal. A promotion should be given to the best person for the job, hands down no questions asked.