If sure post isn't going anywhere. Then the post office need to spend billions of dollars and invest in bigger cars and or more employees. I don't see that happening. I do however see sure post disappearing.
This is a rather sparse post with much left unsaid.
This is what I see based on your base assumption:
Base assumption #1 : USPS refuses any package from UPS and FedEx that does not fit through a coat hanger and weighs more than 3 pounds.
1) Volume goes back to UPS and FedEx Ground and number of Driver jobs at both companies increases.
Base assumption #2 : USPS does not renew their contacts with FedEx and UPS for SmartPost and SurePost.
2) Most packages will go to FedEx 60% or more and UPS Feeder and inside jobs will be reduced while the UPS driver jobs increase.
Base Assumption 3 : SurePost is eliminated by Teamster/UPS contract
3) All packages go to FedEx and UPS Driver positions are reduced as well as Feeder Drivers and inside employees. UPS will lose some packages at the other service levels since some compamies prefer to only have one carrier that fulfills all their transportation needs.