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That's possible as well, if those online stores are going to integrate bitcoin as another payment scheme. But we can't discount the fact that Amazon as the biggest store in the globe, if decided to adopt bitcoin as well will be huge for the market.
We have a lot of stores already accepting bitcoin around, but still I'm guessing it's a small percentage. But if Amazon will lead the way then we might see small ti mid tier online stores also accepting bitcoin to follow Amazon's lead. So it will be a top to bottom effect.
Even if they are the biggest store, if people work together we can destroy their profit, I think that the Wallstreetbets fiasco is an enough proof that we can do it together. Sure them leading the way is good but this could spell disaster for the companies that are selling their products on Amazon because they would get overshadowed by the announcement of the bitcoin integration, as you have said they are the biggest of them all. And that top to bottom effect meant that the one at the top will get the biggest benefit while those on the lower will be fighting for crumbs and droplets.