While it is true that the volume of mail has been declining for years now, the biggest issues currently facing the Postal Service have more to do with political incompetence than technological disruption.
What then, does the USPS do that is essential to the functioning of modern society? While it certainly employs a lot of people who need jobs in post offices and on the street, the basic answer is "nothing."
Sacrifices will be required of the Postal Service, but hacking away at its core strength will not solve the problem. Instead, it would tear down a network that has taken more than two centuries to build. Americans should demand better.