There's nothing stopping an increase in competition is the problem. Sellers want the visibility of Amazon's platform with the benefits of rolling their own. There's nothing physically preventing a merchant alliance from forming its own platform, merchants (that aren't Walmart) simply don't do this because they know they're still getting more value by being on Amazon instead. Online retail isn't new or novel at this point, merchants have had all the time in the world to figure out a more favorable system.