It shouldn't be surprising that DL is putting D1 on JFK-SEA/SAN/LAS. Those identical city-pairs have recently or will soon see Mint. DL introduced D1-lite service on JFK-SEA around 2014, and quickly ended it, so there is reason to be skeptical about the viability of that, let alone SAN and LAS. Nevertheless, kudos to DL for the willingness to take a long cold bath on those routes. (Also kudos to B6, which has repeatedly poked DL in the eye -- we still remember Song.) But IMO, this really has more to do with posturing at JFK than "winning" on the West Coast.
UA's response? Nada.