What happens when EBT cards and Social Security transfers stop flowing; when traffic lights and the neighborhood power grids cease to function; when TV, radio and the Internet are no longer occupying the brains of the masses; when hospitals have only generator power to function, and people are dragging their sick/injured relatives and gang members to the ER; when gangs realize there is zero citizen potential to call for the police in order to stop raids, etc?
There is little doubt that social unrest would quickly occur. But it would take much more than that to get a 90% mortality event.