Multiple POD's
It would take multiple POD's from OTL to stop suburbanization. Yes, it could have been stopped after 1945. After a depression and war that diverted resources, housing was in short supply and it would have been possible to market multi-family homes as a different version of the American Dream; it just did not happen. The interstate highways did not cause urban flight since they were not established until the late fifties and did not become part of most people's lives until the sixties.
The key to keeping the cities strong is to keep railroads and steel strong. Since Americans endured rent controls and commodity rationing during WWII, they would have accepted central planning; it just did not happen.
My POD's:
1. Build the interstate highways so the main routes go around the large cities, not through their centers, much the way I-80 goes around the center of Des Moines, Iowa.
2. Insert this technological POD: The Kroll process for extracting titanium metal, developed in 1946, does not happen until the late sixties. The result would be no jet engines and no jet planes, permitting a railroad-based postwar construction boom. Fast trains and a new railroad infrastructure would be too well established to be shut out by jets that first appear in the seventies. (This scenario has political/military ramifications that might constitute a thread of its own.)
3. Divert the jet plane resources to putting air conditioning into multi-family housing. In OTL, a/c was too expensive for single family homes until the sixties. After all, the Empire State Building was air conditioned around 1950.