The solution
S76Heavy
Every time this topic is raised I put forward my simple and easy to understand solution - which is then ignored by almost everybody!
Seniority should be based on the initial issue date of your Air Transport Licence. Thus when you join a company you fit in to their list below all others with older licences and above those with later dates. For the guys with shiny new licences that would be at the bottom of the list and for the more experienced further up amongst their peers. Changing seats would not be a problem, if seperate Captains & F/O's lists were used, as you would still be in the correct order relative to your colleagues on that list. Changing companies would be less of a daunting prospect as you would not "lose" seniority in the move.
To keep the ex-mil pilots happy they could have credit for the time spent before civil flying - say the date of their first operational squadron......