Spent about a year loading and unloading at the Trabue UPS hub in Columbus back in the 90's. Everything you say is true. Unfucking the belt is something that is likely to be strictly a human endeavor for the next 20+ years.

There's no way automate it with the current hub architecture and today's technology. You'd literally have to have a stadium-sized package 'cache' where all package could be unloaded to, some machine vision doohickey to characterize every package, then have a solver identify the idealized load configuration, then picker bots go out and start streaming packages in order so that some high end robot could load the trailer with 90+ percent packing efficiency (which i believe we achieved as loaders getting paid $8/hr).

Then you'd still have to leave the last 8' of trailer space to handle the goats heads and exhaust pipes and all the other crazy shit that people just slap a label on and make someone else's problem. Maybe you could find a way to use air bladders or some kind of encapsulating foam to handle that, but mostly you'd just have to have a floater running from trailer to trailer to load that stuff.

(internet fist bump btw)