Aworld where the majority of a generation died would not be able to support the specialist technologies, skills, or energy production required to maintain the present level of technology or our large cities. The urban population are interdependent specialists, reliant on an agricultural hinterland. That hinterland, in the case of mega-cities, is global.
The highly urbanised and controlled world of Blade Runner could not survive an apocalypse. It could arise by drift, by the acquiescence of the people to the impoverishment of their lives and environment. Some might argue that this drift is happening.
In a former life I worked in the oil industry and I know how much effort, specialist knowledge and manpower is devoted to seeking out and processing oil products. A Mad Max world simply could not produce the fuel for crazed gas guzzlers with a small population struggling with endemic violence. Witness the standard of living in the conflict zones of the world.
I suggest that we should revert to a more rural style of living like that prior to the industrial revolution, with whatever technology the few surviving scientists and engineers could reconstruct. With many fewer people to feed and house this need not be a terrible world of privation and fear. Life would carry on without computers and the internet! The gravest loss would likely be in the area of medicine. People would reorganise their societies, with probably very different views on justice and politics than are currently the norm.