More broadly, what does a restructuring bankruptcy get Delta (or AA or UA)? The last round of bankruptcies allowed the airlines to discharge their pensions onto the PBGC, so there are no pensions left to discharge. DL has only one unionized workgroup (pilots), and owns most of their (older) aircraft. There is the potential of getting out of the 717 leases (held by Boeing Capital I believe -- can be bartered for a new aircraft order), or returning some newer leased aircraft, but that cost savings alone probably is not worth bankruptcy.
A Chapter 11 filing anytime soon by any one of the Big Three legacies would confer minimal upside, and likely prove counterproductive. Quick example on fleet: During the UAL bankruptcy of the early/mid 2000s, UA hoped to largely eliminate its high-cycle and increasingly inefficient 747 fleet, and use 777s on its longest routes. When UAL sought to renegotiate its 777 leases, the lessors balked, and actually took back a chunk of UA 777s for re-marketing elsewhere, leaving UA even more dependent on 747s until the 2010 merger with Continental. The same could easily happen in a hypothetical early 2020s DAL bankruptcy where the shiny A350s and newest A330s go bye-bye, leaving the airline dependent on the older A330 classics and 767-300ERs.