Everybody assumes everything is a zero-sum game. Either the suburbs must prosper, or the urban core must prosper. There is no middle ground. Joel Kotkin seems to be a pretty smart guy. But he doesn't seem to understand that very few New Urbanists want the suburbs to decline. They just want the suburbs to give up their near-monopoly on transit funds in most of the country, and for zoning laws to allow multi-unit development in areas where there's demand for it.
He also has a lot of trouble understanding that "detached, single-unit dwelling" is not synonymous with suburban. To my knowledge the entire Detroit area has maybe six Residential blocks where most of the lots are multi-unit dwellings, so by his definition everyone in the entire Metro region lives in a suburb.