Stopping marketing mail wouldn’t change the number of accelerations per mailbox. USPS would still need to check each stop for outgoing mail. The only difference would be in weight carried.

USPS would still need to check each stop for outgoing mail.

No they don’t, that’s what the red flag on the mailbox is for. Everywhere I’ve lived, if you don’t put the flag up and there’s no incoming mail for you, they don’t stop.

Depends. Where I live outgoing mail goes into the closest blue USPS bin. And given that most days all mail I receive is slop, removing the slop would remove the need to come to my house.

Of course, where I live the USPS person stops in a general area and does all the outgoing deliveries on foot, but it's conceivable that some days an entire block may receive no incoming mail. Also, we need to take into account things like fuel costs for planes & such throughout the entire supply chain.