A mass-affordable Los Angeles is geometrically impossible. The single-family house and car culture are central to what it means to be "LA" and there are fundamental scaling limits on those.
Those things are scalable -- LA's light rail network is one of the biggest in the country and it's incredible efficient. Over a million people (of a city of 4 million) use LA Metro on a daily basis.
There's no reason LA's fundamental character as a city would change if we allowed duplexes in already dense neighborhoods or as much BRT as possible. But instead we've prioritized voices like the Bel-Air Homeowners Association or some NIMBY idiots in Santa Monica to make policy decisions and are stuck with environmental review on projects that demolish, say, 4 parking spots to build 40 affordable housing units.
In other words it will no longer be the same LA, but rather LAtropolis .. so gp is right about the physical limits.
No one is coming for existing single-family homes -- they can peacefully co-exist alongside properties that house 5x the amount of people.