From a US-centric perspective, yes, but outside US, I doubt it.
US e-commerce market is huge but it only represents a fraction of global e-commerce and Amazon’s global e-commerce business is not even the largest in the world or at least not yet. E-commerce is growing fast in developing economies and Google has a strong position in most of them. (Except in censored internet markets like China.)
In my opinion, Facebook is the biggest threat for both Google and Amazon (again, outside the censored internet markets). It does not take a huge leap of imagination for Facebook to expand their search engine capability and open up FB Stores and let their huge user base buy directly from thousands of merchants who would be happy to sign up for FB Stores. And of course product review can be filtered between in-network connections versus out-of-network users.
Now that is an interesting proposition, except if users will consider it as FB overdose.