Fedex does shipping. Only shipping. Not retail. Why would retail customers ever switch to using Amazon for logistics, and pay their competitor? Amazon does enough business to have their own skeleton logistics network, but it'll never have the full network Fedex or UPS has; it simply doesn't have the retail margins to pay for it.

Fedex and UPS will do just fine. Amazon is going to have to up its game eventually; investors will tire at some point that Amazon retail doesn't make anything, and is propped up by AWS.