Earlier quoted context omitted.
All of the other major delivery services in the US (UPS, FedEx, DHL, Ontrac) are horrible and far worse than the USPS. And without the USPS to compete against, they'd be even worse.
To say nothing of the fact that other delivery services frequently rely upon USPS for last-mile delivery in more rural (read: less- or un-profitable) areas, and those areas would simply not be served by a privatized postal service.
But even setting these things aside, USPS is almost always a better experience as either sender or receiver than any of the other competitors in the US.