The question is, at what point does the hacker threat turn from a minor annoyance like this, where for most people life is back to normal in a few hours - days, to a serious threat to our civilization -- can they keep the network down long enough that food and fuel production / distribution is affected to the point that 1%, 10%, 50% of the affected population dies? Can they permanently destroy so many generators / wires / transformers so that available repair crews and equipment stockpiles are exhausted and entire towns are without power for months? Can they send massive over-voltages over power lines to destroy all plugged-in electronic devices in every home and business, and create widespread explosions and fires at the push of a button?
[1] https://en.wikipedia.org/wiki/Northeast_blackout_of_2003