Stores and banks aren't stocked or staffed for turning off e-commerce. Banks hate when customers walk in the door instead of doing everything online. And the stores woods have to find a drastically different equilibrium without internet advertising or orders. It would be absolute chaos.
He shuts down major backbones, effectively turning off the U.S. Internet. At the same time, he releases a story to the media and telcos about a major attack on the backbones - a virus, whatever. There's panic and chaos, as the country grinds to a halt.
He uses that to declare martial law.
After a sufficient time (a small number of days, or even hours), he turns the backbones back on, claiming experts have fixed the problem. Citing martial law, he puts restrictions in place on the U.S, Internet, as dictators do.
We now have severely limited access to real news and information, martial law, and a dictator in place. American Democracy is dead.
Vote Democrat. No matter how much you think they're the same, they aren't.