Many will say this, but often it gets lost in the overall value of the deal presented:

The longer Delta stalls, the more new hires and junior pilots that can be wooed by the lure of a signing bonus that could/would be bigger than any retro they'd otherwise get. The more distant the memories of past VEOP and retired pilots become. The bigger the "signing bonus" the more people that will be swayed by the overall deal in the end and an otherwise great contract that doesn't have full retro, but some significant recognition of the delayed negotiations will pass.