DL has 5 763s in the JFK-LAX schedule by this summer and DOT data shows that DL is carrying tens of thousands of pounds of cargo per day on JFK-LAX, most of which was carried by AA - part of why the 762s made some sense. Given that the passenger airlines fly frequent schedules thruout the day, they have an advantage over the cargo carriers. And the NE-California cargo market is huge. DL doesn't even intend right now to use 767s to SFO.
DL's execs on the latest earnings call noted that DL is the number one carrier by seats in the JFK transcon market and that likely couldn't be sustained just with 757s. The 767s are also used to connect cargo to/from DL's Asian and Australian flights.
The irony is that AA also flies 763s but somehow the idea of converting some of them to the same configuration so they could continue to carry the cargo market apparently wasn't an option they wanted to presume - likely because the 767s that DL uses have 200 economy seats - a market in which AA says it is dramatically reducing its presence because it doesn't want to compete against B6 and VX' low fares.
As I have said so many other times, DL is not afraid of taking on low fare carriers and DL's ability to limit B6' growth in NYC is directly linked to adding seats in markets like JFK-west coast and the Caribbean that AA has left or has reduced its presence.
You can't selectively serve markets and then be surprised when one day you are relegated to 2nd or 3rd tier status in that market.