I think Amazon can adapt to embrace blockchain, even. Suppose there is a decentralized 'like amazon' space on the internet... what's to stop Amazon from making an account and offering it's entire catalogue on that decentralized blockchain?
Just like when the internet arrived you had some companies who didn't hop on that train and others who did. 'The internet' as a whole certainly hurt retail businesses, but the smart retail businesses now how flourishing online stores.
I think a similar thing can and will happen to blockchain technology. Some companies will vanish because they don't adapt but others will survive and perhaps even thrive.
If you read my INS ICO Spotlight it has a section where INS has identified market demand and supply drivers for e-commerce businesses. When we went from brick and mortar to the internet those were just developing and continue very strongly to this day.
Going from a centralized to decentralized system IMO is not as big of a deal for e-commerce consumers. Price, ease of use and convenience is. In other words, consumers don't really care. It can almost be invisible to the user. If I show someone steemit without mentioning that it is run on blockchain - do you think the person will like it more or less after I tell them? I'm guessing they will mostly be indifferent - the main thing they care about is what value the service can bring them.
I feel blockchain will have different impacts on different industries. Right now i'm not yet confident it will have a large initial impact on e-commerce.
I think a lot is determined by cost as well. If decentralized ends up providing the same product or service (more or less) but at a cheaper cost, that will be a main driving force for people to go decentralized. After all, it was one of the selling points of Bitcoins -> no middleman to take a cut in terms of transaction fees, so it was cheaper than traditional methods of sending.
That went out of the window ofcourse when the fees got high, but still it's important: the blockchain variant needs to be better in some way, and cheaper is one of the more obvious forms of better. If it ends up being only slightly cheaper, then consumers may end up using centralized platforms after all... there are some benefits to that too in terms of provided service and warranty etc.
But yes, this uncertainty is why I stay away from 'decentralized ebay' projects and such. No guarantees they're actually going to conquer the userbase.