The point is that there shouldn't be any suburbs !
They exist because the Cities capacity of quick changes started to decline. Most of them started in the 19th century.

We had some speeches in the French Assembly in 1840 that tells of African villages at the doors of Paris (no African people there, mostly from center of France), that it was a shame... That we should build transient cities (Cité de Transit) to teach those people about how to live in the City... On century after we did build them !

Maybe that's why I speak of the "drift" of the City :-)

"What can I do, What can I write, Against the fall of Night". A.E. Housman