The same as it's been since the 1890s [1]. You move further and further out, because that's all you can afford. When you're rich enough to upgrade a bigger place, you move again, and everyone else does the same. Then some suburbs realize this is a huge drain on their budget, so they attract some retail or office space or manufacturing, and within a couple years they're like Irvine or Tysons or Bloomington -- the suburbs that have made it. Everywhere else accumulates the same problems that inner cities once did, but without the potential to improve.

It's hardly something you'd call sustainable, but there's many metros that have yet to be built out -- there's always another Boise [2].

[1] https://en.wikipedia.org/wiki/Streetcar_suburb#History [2] https://news.ycombinator.com/item?id=16898200