I see that you are a DL and AA (and WN) flyer. Have you ever flown UA? I started flying on UA in 1996, when I moved to San Francisco. Since 2008, I've lived in Seattle (and flew with AS and its partners from 2009-2011). I still fly AS, AA, and DL when I get rerouted for IRROPS or if I have no other reasonable option. All three airlines offer service that is superior to that of UA, and DL's hard product is definitely better. UA's operations are poorly managed, the employees are bitter and frequently unpleasant, and SFO is a headache of an airport. "UA is doing exactly what it needs to" to spiral towards a smaller market share and (eventually) bankruptcy if things don't turn around.
I spread my flying among the big four domestic carriers, so I do fly UA when I can and tend to gravitate towards them among the legacies. The hard product is solid, the network is wide, and the people have been excellent. They earned my good standing with them. This industry naturally offers very little differentiation. One simple variable such as the on-board crew can determine a relatively good or bad product. No legacy carrier offers a universally better product than another and likely never will.
UA is making huge money right now. And with so little competition left, the threat of shrinkage and bankruptcy has never been smaller for them. Their current strategy is to strengthen their hubs, and that's what they've been doing at SFO. It's a gold mine that AA and DL wishes they had.