I don't think this is Amazon's current plan, but removing a large revenue stream from other shipping companies and then growing their own shipping operation on

I don't think this is Amazon's current plan, but removing a large revenue stream from other shipping companies and then growing their own shipping operation on their captive business could end up driving one or more of the current incumbents either out of business or, more likely, to merge. With less competition, overall shipping rates rise, magnifying Amazon's advantage, and possibly allowing them to offer their infrastructure as a general service. Voila, another profit center. This is kind of where Amazon's cloud services came from.