Assuming we would have driverless cars, you don't have to worry about a gigantic shell of metal that is expensive, will break down at inopportune times, requires constant maintenance, copious parking space at any destination of your choosing, insurance, contributes to crappy air quality (thankfully reduced/solved by using electric power), and for which you have to spend hours shoveling snow if you are in a colder climate. There are tons of other problems if you focus on human-driven cars: drunk drivers, the need for designated drivers, comparatively high risk of being killed or maimed, having your kids nag you or someone else if they want to experience the world outside of their neighborhood, driving course costs (oh wait, in America we don't have to pay... in Europe, well, I hope you have several thousand euros). There are so many issues that they greatly outweigh the ones in a properly-made public transit system. I see cars as being useful for out-of-city excursions (but not inter-city, just take the train) or for hauling things, but these situations are not common enough that every single adult needs to own a car.
We don't actually have to tear down and start over, and I think you underestimate the North American will to destroy old buildings. We just need to use existing buildings more intelligently, turn asphalt seas into courtyards, allocate green spaces and keep residental and commercial buildings close together. I don't expect people to actually do any of this, particularly in the US where it would be seen as a communist plot, but I'm happy there are many places in the world where there is no need for urban replanning/reorganization on a grand scale.