Cars are not super cheap, and in fact are very expensive when you start to account for public costs and externalities. Especially in regards parking, building car infrastructure is expensive and space inefficient, and nowhere near the amount built today would be built without laws mandating it. Moreover, highways really aren't cheap either, and Houston is suffering under the financial weight of them.

Obviously since sprawl exists now it's hard to get rid of it; a city would be hard-pressed to go to the suburbs and tell them that they won't receive services anymore. However, as Detroit has shown, sprawl is definitely a liability if the city ever has any sort of period of decline. That certainly hasn't stopped Houston (and many other American cities) from acting as if that could never happen, and I suspect that the next 50 years will reveal the weaknesses of the model. I hope to be proven wrong, but that would require climate change to magically disappear and resources of all kinds to stay abundant, which seems unlikely.