Correct my if I'm wrong, but don't a lot of the ultra-dense cities have some form of areas of makeshift or very unhealthy living situations? Sao Paulo was a city like this, as are many in ultra dense places in Asia, Tokyo excluded but they built down as well as up and routinely demolish relatively new buildings.

California really does have a major problem now with wildfires and encroachment. I was amazed how fast the Camp Fire moved (an acre a second burnt at one point). There's just no way to really escape something like that. This is why I think building down as well as up is a good idea for many cities, Californian cities may have dinner different issues due to their tectonic activity but it should at least be explored.

I agree with your point regarding density in general. I always find it silly when people say America doesn't have space for more people. We have entire states with less population than some cities. I think improving infrastructure between the "flyover" states and coastal states might be one improvement that's overdue, but I don't know what the best solution would be off the top of my head.