This has been discussed on the DL forum but a few things to note:
DL's mainline CASM based on the latest quarterly reports (which will be updated for both DL and UA within a couple weeks) was over 7% lower than UA's mainline CASM. DL's consolidated CASM, including regional carriers, was about 5% lower.
DL's CASM will be going down fairly significantly over the next year as the 717s and 739s enter the fleet and replace older aircraft. Yes, UA is replacing older aircraft as well but analysts seem to believe that DL's CASM reduction will be more significant in part because UA continues to face increased costs associated with merger integration while is gaining personnel efficiencies because of their fleet renewal.
DL's hubs at MSP and ATL are far larger than UA as hub carriers at LAX or SFO. The strongest hub carrier on one end of a route almost always carries the largest amount of traffic and has a revenue advantage which also means they can sustain competitive battles longer.
DL is in stronger financial shape than UA.
All of this may not be near as significant if the real target is Virgin America who is likely to be most hurt by all of this expansion at SFO.
Long-term UA could be better off if DL's actions help put more pressure on VX.
Further, DL's expansion at SFO also seems to be targeted at positioning DL as the 2nd largest carrier at SFO. Based on the combined AA/US traffic at SFO this past summer, DL would be #3 behind UA and new AA among network carriers. But DL is growing SFO while AA and VX are both shrinking, based on current schedules nor next summer, which means that UA could end up with DL as a stronger competitor at SFO but a weaker AA and VX which might be more beneficial to UA in the long-term.
You may or may not agree but those are some perspectives which will play into what is going on in the market.