Delta has nothing to offer that United can't get on it's own. It would make more sense to let DL die and pick at the pieces than add all the headaches of a merger.
Sort of, Fly. Delta has no overseas routes that aren't open-skies (other than one NRT flight). Slots and gates at LGA and DCA (which DL has a lot of) perhaps aren't as valuable as they once were. Delta's airplanes are nothing special - UA could order them from Boeing if it really wanted more airplanes.
But none of the above is what's gonna drive the next round of mergers/consolidations.
What United needs desperately (and what AA needs as well) is a larger concentration of Y, B, H and K fare payors than they currently have. Well, and more F, C and J than they have now, too.
And one source of more high-fare payors is the top 50% or so of Delta's customer base. If you wait for DL to die, you stand a much smaller chance of capturing that revenue. And United (and AA) needs that revenue. You buy the airline, comp all the frequent flyers to the equivalent UAL Mileage Plus level, convert their outstanding mileage balance to Mileage Plus miles, and odds are you'll keep them. Unless, of course, they have bad experiences that drive them to another legacy.
Since DL is in Ch 11, you don't have to take all their aircraft or employees or other unwanted assets. And you don't have to take on all their liabilities, either. Just like AA's purchase of most of TWA, UAL can pick and choose what you want. You take the valuable components and abandon the rest (let the creditors fight over the liquidation of the unwanted crap).
Same thing with AA and Northwest.
When all is said and done, UA and AA probably divest some of what they bought to satisfy the antitrust division and neither is as large as the combined separate airlines. Consolidation isn't gonna be about getting bigger - it's gonna be about dumping your cheapest fare payors and replacing them with the top half or so of the pax of the acquired airline. Total revenue would likely be much, much larger (that's what happens when you fire your most unprofitable pax and replace them with other, more profitable pax). Some hubs would close (CVG, SLC) and some others would be right-sized.