Plus a broader tax base vs relying on Prop 13 property taxes.
The reason you think more transit will be needed is because you are assuming that separate business and residential districts are required. However there is no particular reason it must be that way. If you can get people within walking distance of work and a few stores they won't needs as much transit. Transit becomes what you need for rare trips.
Much of SF is 1 story buildings. You can absolutely convert those to 3-4 story buildings and get 100% more people in SF. The first floor can be restaurants, schools and grocery stores. In this new SF, like in Tokyo, you can get a flat for $600/month.
Think of the Mission, North Beach, Noe Valley, but building that in the Richmond and Sunset. Everyone wants to live there (see the current prices), why not build more if it? Why not build a bunch in Oakland?
People can just switch to scooters and bikes and there is tons of room on the streets. I remember a NY transit authority guy at a conference, he was just amazed at how much wider our streets are. There is tons of room.
Tokyo exists, it is a real big city with low rents and a great quality of life.
> You can’t just build taller buildings and dump 50% more people in a city.
Tell that to NY.
Uber/Lyft and now scooters have already 10x improved transit in SF. Living without a car and even without buses is totally possible now. The cost of Lyft 10 times a week compares favorably with a car, and scooters/bikes are much cheaper. See http://www.rideordrive.org/
Improving caltrain, muni and bart would be awesome, but there's plenty of transit to handle much more housing.
So "moar housing!"