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.
Walking barefoot is a lot cheaper than cars (and not as stupid as it sounds - your feet are made to walk barefoot), but it is so much slower nobody considers it reasonable anymore.
Cars (and by extension the policies that enable car dependence), have an extremely high externalities. They require vast amounts of public spending, lower land productivity, make areas more flood prone, damage the health of those living near car infrastructure, contribute to climate change, are traffic, drive suburban sprawl, and kill more Americans than guns do (crashes alone are just 0.3/100k fewer). The reason 'nobody considers it reasonable anymore' to walk are those policies that build for cars. In places with less car-oriented policies, people walk places. It has nothing to do with maximizing the speed of the vehicle you're using.
I think sprawl would happen anyway, cars are ultimately cheap and convenient (as % of budget large, but still affordable) and by the time traffic is a problem you have sprawl that transit cannot easily be retrofitted to.