On the other hand I know that we are overtaxing the world's resources and short of some massive changes in our system many middle class folks in the western world are going to see a drop in their quality of life as globalization continues and our living standards average out with those of people in developing nations.

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.