HaM
Your logic is too simplistic, 12 years as an FO maybe, but after 6 years you maybe half way to command but nothing like half way up the FO seniority list. Why?
Well most FO's start for 4-6 years on shorthaul, and indeed right now, a 4 year SFO on SH will be in the top quarter probably on the Airbus status list. When he transfers to LH after say 5 years he will be 80-90% down the LH P2 list.
In SH because of the similarity of the trips, anywhere in the top half (ie 3 years service) will see you getting a good bid result, but to achieve a similar bid result in LH you will probably need 9-10 years service.
I say this having done 15 years, 6yrs SH P2, 2 SH P1, 4 LH P2 and 3 SH P1. I have the seniority for all but 747 command and should get that bid next year (all things being equal).