It would be a stupendous – 10-to-11 digit – waste of money. There already exists a perfectly good, under-utilized airport across the bay that is equally or more convenient for 80+% of SFO’s market, that has space for new terminals and runway capacity for decades of growth. The only reason we’re having this conversation is because we don’t have a regional agency managing capacity in a rational way. So we have a tragedy of the commons situation, where it’s in every airline’s individual interest to maximize their footprint at SFO and we end up with far more capacity than can be handled anytime weather conditions are less than ideal. Just add a cap at SFO and let the market adjust. Some CRJ-200 flights would end, some traffic would gradually shift to OAK (and SJC) and everything would be fine.
I love flying out of Oakland, but I’m dubious an expansion of OAK or SJC (either via a flight cap or as championed by a joint airport authority) is going to fix the issues at SFO short of a government decree forcing certain airlines to use OAK rather than SFO a la Beijing’s new Daxing airport (which obviously won’t happen).
OAK is much easier to get to downtown SF than it used to be with the newish AirTrain, but it’s still a pain to get to from the Peninsula—which is booming with tech construction as the center of Silicon Valley tech continues to shift north as tech companies build out new facilities on the Peninsula and in SF itself. That same shift northward also impacts SJC’s prospects for expansion (despite its impressive growth over the past few years).
The bulk of the high yielding passengers are still closer to SFO than OAK. And SJC is just as hard to get to on 101 for much of the Peninsula as SFO is. The airlines are chasing deep-pocketed tech workers in Palo Alto, SF, and pricey suburbs like Hillsborough, not the (relatively) middle class folks living in Concord and Morgan Hill. Your proposed cap will help expand service at OAK and SJC, but the airlines are still going to put as much service as possible into SFO as they can because that’s where the money is.
And even if a cap forces a large chunk of domestic SFO traffic across the Bay or down south (and AA, Alaska, and Delta aren’t going to let UA further strengthen their hub without a fight), you’ve still got some of the same issues with runway restrictions at both OAK and SJC. They have plenty of capacity now, but OAK would need to realign the northern complex to give themselves two real parallel runways (and fill in wetlands) if they wanted to host a true competitor to SFO while SJC is hemmed in by highways and development. Expanded terminals at these airports would go a long way to taking strain off SFO’s future growth, as would better mass transit connecting the airports to the outlying parts of the Bay, but it would be expensive if not politically impossible to reconfigure those airfields to make either one into a true competitor to SFO.
Best case, I’d expect OAK and SJC to absorb some of SFO’s projected growth while SFO remains the cluster it is when low ceilings force it into a single approach airport.
And with regards to some of the comments in this thread asserting locals don’t care whether SFO is a disaster area whenever weather rolls in, what a ridiculous take. Plenty of Bay Area residents like me would prefer our travel plans (and the US air transportation network as a whole) not get disrupted every time there’s a rain shower in Millbrae.