Completely agree on your suggestions for urban areas.
However, not everyone wants dense housing... Suburbs exist because people do not want to be that close to each other. Some people want to own their own yards/property/houses, have open space, and are willing to trade driving significantly more (with its associated dangers) for these features. People are going to create the kind of environment they want to live in, and we should figure out ways to make that environment better/safer, rather than dictating that everyone must live in (well-planned) urban environments.
The dependence on cars creates very unhealthy political climate (wars, climate change denial) and creates obesity. Unwillingness to comprehend the issues, created by cars is actually a cultural defect of the nation. OTOH, preference for suburban lifestyle is also a purely cultural phenomenon, and is caused by the fact people prefer living arrangements the grew up in. I grew up, for example in a dense Soviet city, and find big house with a yard absolutely repulsive.
Frankly I think you'd find a lot of people would happily move into denser areas if they were more pedestrian friendly.
And so much of the sprawl in the US is not about residential housing taking up lots of space, but about separating shops and other essential parts of infrastructure from housing in a way that is only tolerable because of the car culture.
E.g. I used to travel to Palo Alto and Menlo Park a lot for business a few years back. I don't have a drivers license, so I experimented with how possible it was to stay at various hotels while walking as much as possible (didn't want to depend on co-workers or cabs to get everywhere, and I like walking).
At one point I stayed in Atherton and walked to/from offices in Menlo Park. People probably thought I was pretty crazy, but to me the distance was no more than what I'm used to, and the "commute" length was shorter.
But what got me was the structure of these - by US standards - quite reasonably pedestrian friendly small towns. If you lived right next to Menlo Park or Redwood City or Palo Alto town centres, a lot of things would be within walking distance. But basics were missing. Such as grocery stores. I soon learned to pick hotels in suitable locations to be within walking distance of somewhere where it'd be possible to actually find a decent selection of food nearby (eating out every day gets boring surprisingly quickly).
On top of the distance, the pedestrian friendly areas were "islands" not linked in any reasonable way. E.g. from Menlo Park to Atherton or Redwood City it seemed to me I had the choice between walking down El Camino Real, including a long stretch with pretty much no roadside lighting (fun walk back late evenings) and no sidewalk (for those unfamiliar, El Camino Real is one of the main roads in the area and heavily trafficked), or taking a huge - 3 times the walking distance or so - (but scenic) detour through back roads past Atherton estates.
(Amusingly, as I tried the backroads through Atherton, everyone I met greeted me as I walked past. It seemed like people assumed I must belong there, because why else would I be walking there? (I'm used to people being more friendly in California than I'm used to from London, but this was a huge step up).)
This is not about density, though density helps, but about a culture where everything is centralised because everyone drives, and where everyone drives because everything is centralised.
Of course there's a place for suburbs, and I'm not opposed to the idea of less dense housing. Such housing can exist in the context of walkable neighborhoods with public transit, though. I think there's quite a bit of merit to firm city limits that prevent sprawl and car dependence.
That said... if someone wants to live in exurbia, he should pay an appropriate amount to access amenities downtown. It's absurd that someone can pay $1500/month for a studio downtown, but only a few feet from his front door, anyone can drive a similarly-sized vehicle at hazardous speeds for free.
Driving is essentially free in the US, despite enormous costs in terms of infrastructure and public space. That needs to change. If car-dependent exurbs must continue to exist, then we should at least make those exurbs responsible for their own costs rather than forcing urban residents to have their neighborhoods destroyed and then pay for the privilege.
Downtowns are dying because parasitic exurbs have both removed tax revenue from the urban core while multiplying their infrastructure/services/health costs. It's unsurprising that residents flee cities that become dangerous, dirty highways.
Some of these cities are now beyond repair, not least for the overarching national trend of capital concentration in Silicon Valley/Silicon Alley, but these reforms would still help enormously. For example, Bloomberg's plan to toll bridges into New York City that was vetoed by car-dependent Albany. It's absolutely ridiculous that exurban commuters are allowed to devastate NYC every day, for free, because they're too entitled either to live in the city or even to commute via less destructive trains.
I would really love to hear your explanation why New York City should be required to spend billions on bridges and roads crushed by automobile traffic for commuters who don't even pay taxes into the repair fund. Much less why NYC residents should be forced to give up enormous swaths of precious road real estate at peril to their health and quality of life.
"I would really love to hear your explanation why New York City should be required to spend billions on bridges and roads crushed by automobile traffic for commuters who don't even pay taxes into the repair fund"
They don't. They can stop doing it any time they want.
I don't think the result is going to be what you expect, though, especially now that no one has to live (or even visit) the city to do most types of business.
Downtowns dying? This is something new. In some cities - yes, but in actually desirable ones (Pittsburgh, Austin, Portland, you name it) - no, they do not
However, not everyone wants dense housing... Suburbs exist because people do not want to be that close to each other. Some people want to own their own yards/property/houses, have open space, and are willing to trade driving significantly more (with its associated dangers) for these features. People are going to create the kind of environment they want to live in, and we should figure out ways to make that environment better/safer, rather than dictating that everyone must live in (well-planned) urban environments.