On second thought, I guess there would be a short term huge increase of sales on eBay, where most Amazon sellers also sell, so I guess "out of business" is hyperbole.
If it took a year to build your service on AWS products, it shouldn't take years to do it again with another provider now that you've done it before. A server is a server, whatever you've programmed to run on those virtual servers will run on identical virtual servers at another company. It's only the parts of your system that interact with Amazon's APIs, and your human systems/policies, that have to be reworked.
And if you're big enough that your systems take years to build, you have a disaster plan already. If Amazon blinked out of existence tomorrow, Netflix wouldn't be standing there with no idea what they'd do to get back online.