^That's all I wanted.
I wish I really had a clear answer on this one. The existing infrastructure of the major Rustbelt cities is definitely an advantage, but I don't think that it alone, or even primarily, will be the saving grace of those cities--otherwise, the larger share of the economic growth we see occurring in the Sunbelt cities, large and small/midsized, would be directed towards the major Rustbelt cities. Also, we should take note that a lot of the Sunbelt sprawlers are currently seeing a lot of quality infill development happening now which is accelerating the maturation of their infrastructure.
Also, I don't think that decentralization has to be synonymous with suburbanization (in terms of form). I think DC is a pretty good example here, as outlying urban centers such as Bethesda, MD, Silver Spring, MD, and the Ballston-Rosslyn corridor of NoVa serve as significant employment centers in their own right, but those areas have been urbanizing for some time now. Of course, it could be argued that this is an anomalous situation given DC's unique circumstances as a city, but it could very well serve to be a model in this regard. One thing that has helped DC here, and could potentially help the Sunbelt cities, is an enhanced level of regional cooperation, particularly when it comes to mass transit, that several Rustbelt metro areas lack due to a plethora of municipalities (which tends to be the result of archaic/restrictive annexation laws) all competing against each other for a piece of the pie.
There are other factors that really play into this and the more I think about it, the more they come to me. Those I mentioned earlier in the thread are simply those that I came up with off the top of my head. Another one that I could throw in there is climate, even though there isn't really a consensus here as to how important a role it plays the growth of the Sunbelt cities.