On a similar note, I also have a comment on flights similar to the previous regarding flights to the San Francisco area:
As the fifth-largest foreign tourism destination in the US with over 3 million tourists/year (as of 2014), it makes sense for a lot of airlines to serve SFO based purely off of tourism. However, to fill a flight completely, one cannot base their market completely off tourism aside from a few markets.
Aer Lingus most likely serves SFO for tourism both from Europe to the US as a somewhat low-cost carrier, and from the US to Europe due to Irish tourism and more low-cost full service to Europe via DUB, seeing as the majority of Irish-Americans mostly live in NE/ the NE USA.
Air Berlin also seems to fit in the same area, considering the population areas of German-Americans in the US to mostly New England, the Deep South, Pennsylvania, New York, and the Midwest (all of which have some flight to Germany by LH or AB).
Air China, China Airlines, China Eastern, China Southern, Cathay Pacific, and EVA Air (as well as to some extent Singapore Airlines for connectivity) most likely all serve SFO both due to tourism and the incredibly large Chinese-American population in San Francisco (I could not easily isolate statistics for Taiwanese-Americans and HK-Americans so they fit in this category).
Korean Air and Asiana most likely serve SFO due to high numbers of Korean-Americans and tourism, and Japan Airlines and ANA most likely serve SFO from Japan due to the high numbers of Japanese-Americans living in California in general.
I'm getting tired of typing a paragraph/airline or region, so I'll simplify the rest in a list:
Most Likely Using Local Population + Tourism for Feed:
-Air India (3rd largest Indian-US population)
-Air New Zealand (relatively large Pacific Islander + NZ population)
-Copa Airlines (connectivity to Panama/Central America)
-Fiji Airways (large Fijian and surrounding island community)
-Philippines Airlines (large Filipino population)
-Qantas (relatively large Australian-American population)
All the rest of the airlines most likely use a combination of connecting feed and tourism to fill up their planes, especially considering airlines like XL Airways France are especially tourism-reliant.
While SFO may be UA's 3rd largest hub and Pacific gateway, IMO DEN should not receive the same benefits, considering it is behind SFO in terms of size, and a good portion of the connections through DEN to Asia can simply be routed through SFO instead, considering most destinations west of DEN are also served by SFO and large destinations east of DEN are as well... UA also manages to feed off of local O+D to support its flights out of SFO, which has a large advantage considering the Bay Area alone has more people living in it than the state of Colorado. While as a connecting passenger, it can be faster to travel via DEN than SFO mileage-wise (ex. JFK-DEN-NRT is roughly 300 miles shorter than JFK-SFO-NRT), it can also be less convenient considering how frequent UA flies to some of its destinations (why is LAX-SEA still a 2x daily ERJ?). It can also be inefficient to fly route such as DEN-PEK, for example, when the majority of traffic that would take UA would rather connect through SFO because the layover is shorter, or take a Star Alliance partner via NRT/SIN/ICN, etc. To me, it seems like some routes to Asia could be feasible, however it also seems like that isn't likely in the near future.