It would actually be very expensive to do. Keep in mind, if you give longevity you are also giving 401k matching, vacation and sick time accrual rates.
Back in 2007 (or maybe 2008), prior to the current FFD workgroup, PSA's MEC spearheaded an effort, that eventually led to USEPA, to try to limit the whipsawing of aircraft back and forth. At the time we were watching CMR tails get placed at ASA and Skywest for Delta flying. In order to prevent something like that from happening within the USAirways system the goal was to work with the US Express Carriers to a) harmonize the contracts so that, longevity aside, it would cost the same to operate an aircraft at any of the properties and b) work on seniority portability so that a pilot group would migrate with their planes, keeping longevity and possibly seniority. While everybody at the meetings was interested in the first part, we pretty much got laughed out of the room (including by the Eagle MEC who was at one of the meetings) for even bringing up the second part. It ended up costing the MEC Chairman his position because it was spun that he was trying to give Mesa pilots left seats at PSA.
Interesting how things change over time.