And traffic isn't a problem if the city is designed right. The idea is that each suburb is an edge city with plenty of businesses. In Dallas, for example, it's not uncommon for people in an inner-ring suburb to commute to an exurb, and when people do commute farther inwards, it's typically people commuting from an exurb to an inner-ring suburb. The long commute form exurb to downtown doesn't happen that often here. Street layout helps, too: we have arterials on a grid system, and all arterials are six-lane divided highways. Freeways are placed at regular intervals on both the north-south and east-west axes to segment the city into bite-sized chunks (just go to Google Maps: Dallas looks like a sequoia because of the way the multiple loop roads are laid out). Five-layer stack interchanges are used to cut down on the bottlenecks in freeway interchanges.
Cities are antiquated, noisy, dirty, and crime-ridden. Suburbs are much more comfortable places to both live and work.
For those of us that want to live in particular cities because of the city itself, supply is far below demand. Some of current residents are trying to keep supply low. That's what we're objecting to.
Why should cities not be allowed to change and have their density spread? The heart of a city is its density and population mass. It didn't become the perfect amount of population when the current residents arrived, never to be changed again. A city that does not change is dead.