What you're describing is the competitive advantage of shoppers preferring Amazon though, at least from how I'm reading your post. If a seller alliance or rival omnicorp can't entice consumers then they need either better marketing or better deals, competing in short. If they can't succeed at either of those in a manner significant enough to challenge Amazon then how is targeting Amazon with legislation actually helping consumers? It seems like most policy proposals boil down to making Amazon less competitive rather than cultivating an environment that results in more platforms capable of reaching Amazon's combination of scale and popularity. If Google was worse at Search there'd be more competition in the space but it'd hardly be a win for consumers unless you change the definition of a win.