Agree with DMORBUST.
Everything he said is correct.
It's essentially "buy 3, get 1 pair free" i.e. take three years of losses on a crappy route and then do whatever you want with it!
If delta can minimize their losses with the 757 over three years, they gain a slot pair potentially worth $20-30 millon.
No idea if this route falls under their JV, but if it doesn't I'd imagine there will be some cheap fares to LHR since DL will have to provide it's own O&D at two non-hub airports.
Could be a rough few years, but then they get another lhr flight from ANYWHERE.