You're right, traffic patterns are not balanced, so the cars returning to the hot areas for pickups will not be contributing to the traffic in the most-congested direction.
Also, SF already has BART, Caltrain, and SF Muni. I'm not familiar with the specific history here, but transit and housing are the limiting factors for where people live in the first place, so the number of commuters will expand when capacity increases. The better question is, what if SF didn't build those systems? It wouldn't be San Francisco today minus the trains, it'd be a totally different San Francisco. So you can't really take the ridership numbers from today as evidence that city couldn't cope without trains.
Also, this was all happening when self-driving cars were not around the corner. It's a different story today.
Except that in a city, all traffic contributes to congestion,there is no single "commute direction".
>Also, this was all happening when self-driving cars were not around the corner. It's a different story today.
That "corner" is decades away, meanwhile, people need to get to work today -- and you still haven't explained how self-driving cars can replace a million+ trips/day on city streets that are already at capacity during rush hour. They may be self-driving, but they still drive on roads.
And don't say "But self driving cars will fix everything! They don't even need to park, they can drop you off at work and drive away". If the cars aren't parked, they are contributing to congestion. And since commutes aren't generally balanced, it's not like the self-driving car will leave Walnut Creek at 8am, drop you off in SF at 9am and then pick up a new passenger to head back to the East Bay - that car is going to be loitering around SF somewhere, waiting to take someone back home to the East Bay.
SF Muni carries 600,000 passengers/day. BART carries 400K (though not all to SF). Caltrain contributes another 50K. How could you possible accomodate all of these people without transit?
Like I said, building effective transit in existing cities is very expensive -- the BART system cost around $1.5B when it was built (around $20B today), but few would argue that SF would be better off without it.