SFO is NOT a profitable hub. It is actually a DRAIN on the UA network. Yes, it is a profitable STATION, but running it as a hub is a big reason why UA is in financial trouble.
A hub can be defined as a waypoint between an origin and a destination. A significant chunk of UA's SFO traffic is NOT connecting traffic, but rather O&D for the Bay Area. As a result, the O&D traffic (which by nature is higher yield than hub-and-spoke traffic since it requires travel on only a single flight segment as opposed to multiple ones) takes up a significant chunk of the available capacity to/from the airport. This capacity has a finite upper limit, based upon the geography and logistics at SFO airport.
It is a proven fact that the North-South traffic along the West Coast is best served by point-to-point service, which rules SFO out as an effective hub for this kind of operation. The concentration of population in just a handful of significant population centers (SoCal, Bay Area, Portland, Seattle) renders traditional hub-and-spoke between these centers unneccessary. Obviously, secondary centers need to be fed by regionals out of these major population centers, but UA's regional service is franchised out anyway.
That leaves SFO as a hub solely for international or transcontinental services. Internationally, it serves as an ideal gateway due to its proximity to the Pacific and its relatively central location along a North-South axis. SFO will always be a succesful hub for this purpose.
However, transcontinentally the same proximity to the Pacific works against it. To be optimally effective, it would require to be fed from the other population centers of the West Coast and then serve as a springboard across the country. However, with so few major population centers out there, all of them can serve in the same capacity - and they do for the most part. They each have a catchment area (including proximal regional traffic) that is more than capable of providing enough traffic for dedicated operations.
As a result, every connecting passenger through SFO is actually taking away a potentially higher yield local passenger. Since there is a finite ceiling on capacity at SFO, continued growth as a domestic hub will DILUTE this yield even more.