Is Amazon going to be a competitor?
Amazon is looking to cut cost and increase profitability. Amazon won't be hauling Overstock, Wayfair, ebay, Best Buy, or any of their competition. That is unless they buy them like they did Zappos.
There is no way Amazons competition would support an Amazon Delivery service to help their competition be more profitable.
Now the down side. There are a lot of small businesses that fulfill orders for Amazon outside of the Amazon DC's. I'm sure those businesses would use an Amazon Delivery service or an approved carrier to take advantage of competitive rates.
Bottom line is I'd be concerned about lost revenue. Lost volume of this size would lead to lay offs at UPS. The company would find a way to adjust and maintain by sacrificing payroll.