Living in a European city and relying 100% on public transit and walking, with occasional biking for longer distances, it’s easy. It does not require 5-10x the population density, maybe just 2-3x, at least compared to San Francisco (which is already somewhat dense in parts). It is not a major shift in lifestyle easier. The tallest buildings in some cities in the Netherlands are 2-3 stories, and those cities still have regular buses, trams, and trains.

There are even some suburbs full of single family homes, with yards, which still have buses every 8 minutes all day long, and long-distance commuter train service every 20 minutes.

Yes the US needs more density for this to be feasible. But San Francisco isn’t that far off on the density that it would needs