The decision isn't made by a trillion dollar 'Amazon'. It's made by a person inside Amazon with a limited budget, whose ROI is critical to their personal financial success. At the end of the day, the decision to compete will be an ROI question, Is it better/cheap enough to roll-my-own, or is it has this outfit done a good enough job that it is better to partner? Outstanding people are a precious commodity, so if the startup is really good, why wouldn't you partner with them? And if mediocre when you are really good, why would you?