A couple of thoughts:
Could seniority be defined by the time that you've actually spent in aviation flying actively ?
What about those who were forced (by Force Majeure) to change airlines and hit the bottom of the infamous seniority list every time and started patiently all over again ?
I strongly believe that command should be given to the most experienced F/Os in terms of hours, variety of flying duties, route structure, number of airfields, weather conditions, they've been through, and number of aircraft types flown.
Dump the seniority. I've personally had it, I want my command after dragging my arse in the right hand seat around the planet for 16 years, mostly flying for majors !