As you outlined yourself Amazon will be using it's delivery services in high density, high margin areas leaving low margin low density areas to USPS, UPS etc. This will def. hurt UPS and Fedex.
TLDR: Losing volume from Amazon isn't so bad.
As I worked support in a UPS hub in Jacksonville, FL, I can confirm that actually, losing some volume from Amazon will help. The facility I worked at was at volume capacity per hour over five years ago. Updating the belt system in the building is problematic due to space available and time lost during the shutdown. Year over year the volume has only grown as more people order more online. The capacity in that hub remains the same which means they have to run more sorts/shifts and the operations start to collide together and so on. A little less volume could be seen as a good thing. As long as revenue holds.
Maybe. You’d have to explain to me though why there isn’t a major delivery service today that has a strategy to skim the delivery cream in dense urban areas. Not an expert in this market but I expect there are costs associated with urban delivery that offset the advantages of density to some degree.