Then we got TV and Newspapers and Magazines started to erode.
And does anyone buy 45 records anymore??
The world changes. For the postal service to remain the same for the last 60 years is ridiculous.
The test is whether the payload is pure information or has an essential physical form.
If it's information, you don't need the Post Office. Letters, bills, checks, photographs, movies, newspapers, magazines, books are all pure information: modern technology can push all of those through the glass fiber to my house.
That leaves stuff that has a physical form. FedEX. Big Brown. Amazon Prime comes to mind. Unlimited free two-day. One day for $3.99 / item.
Right now, the Post Office subsists on revenue from junk mail (physical spam). That's not going to last. If it wants to remain relevant, it has to outdo the other carriers in the physical delivery of stuff I want.