I said PRIMARY form of gainful employment. Of course there were commuters before that, in substantial numbers. They and their offices drove a lot of the SF economy. But they weren't, to a first approximation, the whole thing (except for services provided to them by the underclass) the way it is now. SP et al pretty much gave up the commute game except for the Peninsula by the 1950s as people jumped in their cars. And SF before the 1960s-70s did have other things going on that supported many workers, including some productive industry (not just locally-oriented distribution and finishing work).

BART was a game changer because it provided high-capacity transit from deep into the East Bay, expanding the "commute shed" and allowing a lot of big offices to build up in the Financial District and SOM. Not to mention the government buildup around Civic Center. In theory, Key System & SP could carry similar passenger loads across the Bay, but only from the near East Bay; BART pulled the car-oriented suburbs into SF's (and Oakland's, to a lesser extent) orbit, and it made a lot of 1980s-90s growth in SF possible. Can you imagine the daily BART load trying to get into SF by car? Wouldn't happen - no place to put all the cars and buses even if they could get across the bridge within 24 hours. Without BART the Bay Area would have a very different land use pattern, probably more ferry boats, and probably a less vibrant (overheated?) economy. SF would look more like a bombed-out rust belt downtown, with a touch of LA parking lots thrown in.