A couple of us (FWAAA is one) who post in the UAL forum saw where this was going when DAL started to expand in SEA in 2012.
Interesting thead here (from post 159 on) that covers the same ground, lays out some of the cost issues at different ranges.
http://www.flyertalk.com/forum/delta...monday-11.html
I actually ran some hard numbers on the fuel cost difference between mid-range (6000 mile) flights and longer flights here:
http://www.flyertalk.com/forum/19480220-post191.html The impact is larger than I think most would expect, and this shows why SFO is a better (400 mile shorter) gateway than is LAX, and why SEA (500 mile shorter) is a better gateway than in SFO. Suffice it to say that that extra mileage tacked onto the END of a flight is very expensive in fuel costs, and its far better to have even a slightly longer domestic flight, provide the long leg is shorter.
SEA has a series of advantages for DAL which I think are in the following order:
(1) the long flight is considerably shorter ex-SEA than ex-DTW, so routing traffic via Seattle results in (a) better CASM, and (b) allows the use of more fuel efficient mid-range AC that are cheaper than getting new planes (A330s) or planes DAL always owns (763ERs) than the old direct flights via DTW.
(2) SEA is able to pull connecting traffic from the entire US w/o a back connection. With ... network and DLs new flights most of the US can be served to asia one stop. This makes Delta's network now accessible to another large group of customers, including at LAX where they are building up.
(3) while SEA has a smaller O/D market than SFO, it also has less competition. For example, UAL manages to fill up a 747 daily SFO-HKG. Yet CX runs two 773ERs/747, and SQ runs a 747/773ER as well. So UAL only has 1/4 of the capacity. Via SEA? Well no one else serves HKG. Delta should get a much higher share of the available O/D market ex-SEA than UA gets out of SFO. I'm not sure with more competition that UA has an edge at SFO
(4) SEA as noted above has far fewer delays. Less misconnect possibility is huge for business travelers.
(5) Delta can poach traffic from UALs hubs (SFO/LAX) as its a single connection, and the travel times, even with a stop will be similar to direct on UAL. UAL though can't poach traffic from the pacific northwest, as it will be much longer back connection, via SFO with its weather delays.
I go to Asia relatively often, and I can tell you that DAL (either in Y or J) via SEA sounds nice compared to direct from SFO on UAL, especially the 744 in Y. And at this point, except to a few cities, I'm looking at connecting onto NH, which is great transpacific, but has bad regional product.
Bottom line, I think that DALs new hub in SEA will steal more traffic that UAL used to get than most folks here realize.