It's not impossible, but people are stubborn and the voices of home owners/drivers dominate the conversation over anything else.

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.

When you increase population density significantly, the nature of a place changes.

In other words it will no longer be the same LA, but rather LAtropolis .. so gp is right about the physical limits.

LA was zoned for ~10 million as recently as 1960. While I think you're right that the problem is probably intractable—who would want to give up their house for an apartment, unless maybe a dense, walkable area appeared overnight?—I wonder if there are legislative solutions, like rolling back Prop 13 (which will probably never happen).

But people don't have to give up single family homes to create density -- we just need to build more of everything else, wherever we can, to increase housing.

No one is coming for existing single-family homes -- they can peacefully co-exist alongside properties that house 5x the amount of people.

I'm not even talking about mass-affordable, I just mean being able to buy a home on less than a $600k/yr income. Do wages and housing prices continue to grow at 20% YoY indefinitely in cities like this, eventually pricing out everyone except the ultra high net worth?

As long as there are fewer homes coming available than ultra high net worth people who want to live in them, absolutely.