The USPS already scans the exterior of every letter they deliver (perhaps a result of the 2001 anthrax attacks?). They let you see the scans online. You don't have to "speculate" about losing mail.
You don't understand what is happening. UPS made this mistake once and is not going to do it again.
For business customers, accepting a package and failing to deliver for the holidays is WORSE than accepting (a la USPS) with no limits and then having it get stuck.
UPS did this to amazon once, and that was enough for Amazon to decide to build out its own delivery network.
Shippers can do all sorts of things to help make things efficient if UPS communicates the limits. Ie, no sales with guaranteed 2 day shipping 2 days before xmass.
So you still think it's a good thing that a private company should be handling important infrastructure and be able to say, on a whim, "Nop, we're not going to handle this because reasons. Sorry not sorry."
How can you say for certainty that they won't do it again? Are you in a leadership position at UPS where you're able to make that assertion?
Anecdotally, I'm getting my neighbors mail several times a month. I can only speculate about how much of my mail gets delivered elsewhere.