Due to the human interface issues, USPS will still have to maintain some semblance of Saturday retail window hours. In the small town where I live, retail hours have already been trimmed somewhat.
Removing Saturday delivery will save in manpower, and reimbursements for milage. To the best of my knowledge, rural carriers only work 5 days a week. So the 6th day (regardless of which day it is) is filled in via a substitute carrier. Those subs could see a drastic drop in hours of work.
But back to my original comment about back office operations... If you dumped Saturday, all the mail that would have been sorted on Saturday morning, will still be sitting there on Monday morning, waiting to be handled.
There may be savings here, but don't expect it to remove 1/6th the operating costs of the USPS.
Mail is sorted throughout the whole week, if anything we might see less hours being worked by the sorters on Thursday and Friday when the Saturday deliveries are sorted out.
Keep in mind only Saturday delivery is being suggested to be dropped out, that's the end-of-the-line. You still have to sort at all kinds of upper levels (does this piece go to a zipcode within this state, or one of our territories, USVI, etc? And so on, until they get to the local USPS PO, which is the one which would handle the local Saturday delivery).
(Not that this disputes anything you said, just pointing out that mail will probably still be sorted the same way its being done right now)