It turns out there's a difference in the way the private sector and the public sector account for pensions. In the private sector, if your actuarial numbers tell you that the pension is going to ultimately cost $1 million, then you have to have $1 million in the bank right now to fully fund it. But for public sector accounting, you can figure that the magic market money tree is going to top you up by 7% each year and so you'd only need, say, $800k right now to "fully fund" that same $1 million pension. The "onerous" USPS pension obligations is really forcing the USPS to fully fund the pension according to private sector accounting. [1]

This explains a lot about pensions. First, having to have the entire pension cash value up front makes pensions extraordinarily expensive: giving someone a $1k pay raise puts you over $20k out-of-pocket for that pension. Consequently, the private sector abandoned pensions because they were unaffordable. The other observation is that, because almost all public pension funds have assumed unrealistically high growth rates, so consequently their funding rates have consistently slipped.

The truth of the matter is that virtually all public-sector pensions are in exactly the same boat as USPS's pension is. It's just that the governments are more effective at sweeping their pension issues under the rug, while they have forced the USPS to have its pension issue sit in broad daylight.

We should be pushing to force the same reckoning on cities and states sooner rather than letting things deteriorate while thinking we'll magically make up lost ground in the next decade.

[1] Edit: As an addendum, the USPS is specific required to 100% fund its pension benefit. This 100% funding requirement is actually legally unique, although a decent portion of the private sector does seek to do so.