Anthill,
I don't think I've played the man at all. My three "definitions" include the likely emotions of some of the players involved - I have deliberately not made any derogatory remarks concerning those individuals who may fit these three definitions; just laid out my opinion of the likely motivations involved.
You are attemting to debate datal/non-datal seniority without considering the emotion of those involved. When it comes down to who gets promotions, believe me, there will always be a lot of emotion involved.
Although you quoted my question in your repsonse, you didn't directly answer it. If you can say "yes, I would be happy for that person to get that promotion instead of me", then you are an incredibly rare commodity in an industry where the general catchcry is "Every man for himself".
I have no problems with datal seniority, other than leave allocation. In Qantas, we were all told that we were "recruited to be Captains, not S/Os or F/Os". Therefore, everyone is deserving of a chance. Those who feel that they would struggle during an upgrade, do not have to apply for promotions, while others who come up for promotion are required to undergo simulator sessions to determine whether they are suitable (or ready) for an upgrade. So the system already has some checks and balances.
The problem that I see with any other form of promotional system is that I cannot think of one other variant that is not open to corruption and abuse by either the individuals or the system. I'm afraid I also can't see the link between seniority and T&Cs.