New, centrally planned cities have only ever been a disaster that quite quickly make apparent the shortcomings of the design professions. Brasilia is a good example of the sort of planned city that would be built. Widespread availability of cars positioned them as the symbols of modernity and so the city was designed around the car, resulting in an entirely unwalkable city that segregates the classes. See also Chandigarh. Barcelona is the main counter example that springs to mind for me.

Every time this is brought up I fail to understand how people think that this time would be any different. Cities are agglomerations of patterns at every scale and the emergent beauty that arises is absolutely impossible to design in any sort of masterplan way. Organic cities are resilient in ways that we can't every really understand, and that's the important element in making a place that is livable for the longer term.