A quick internet search revealed that, for UPS anyway, no single customer accounted for more than 10% of Brown's total revenue. Doesn't seem like Amazon is that critical to their survival and don't forget that FedEx and UPS were around and thriving long before Amazon and Google came along. Doubt Amazon could realize cheaper shipping rates running their own shipping company; they don't have the economies of scale that Brown and Purple have so they'd also have to ship non-Amazon product. Not sure why a glorified Sears catalog company would want to morph into an international shipping company. Seems cheaper and with less risk to just outsource the majority of its distribution.

Disclosure-I was a science major.