Megacities are immensely fragile because complexity is only prevented from descending into chaos by the order provided by massive quantities of energy. The power outage in Manhattan is a reminder that civilization cannot function for more than a few hours without electricity.
What will happen in a 5G, Internet of Things world if the whole mind bogglingly complex edifice rests on the tottering, variable and fragile basis of Green Power? In such a civilization a hundred percent uptime would be a hard requirement for many systems.
In a less technological era the fall from the few appliances to the state of nature was short. Running out of candles in 1802 would be inconvenient but not disastrous. But in our complex world running out of power means losing comms, refrigeration, water and maybe even the elevator in a 60 storey building.