And if lots of items became available on other sites, but not Amazon (as per the theory being propounded), then Amazon would no longer control 35% of all US e-commerce.
This is a classic coordination problem: you need all the sellers to leave together so that individual merchants don't take a loss. And coordination problems like this are one of the main areas government action can help!
Also, I'd imagine that coordination would probably leave them vulnerable to their own set of anticompetitive allegations (i.e., some variant of cartel behavior/price-fixing). To the extent anyone acts, it would probably have to be government.