I literally used the word "should" in the portion of my post that you quoted, but I see you were a little eager for electronic head pats.
There is no reason, in principle, why USPS can't eat up enough of the UPS/FedEx/Amazon market-share to at least stay afloat as a business without having to resort to the business tactics that people are most concerned about. (poor working conditions, anti-competitive behavior, etc.) But they aren't. They are chronically under-performing, and to me that is evidence in of itself that they are not serving their stated purpose right now.