The point of having a direct competitor to Amazon is to limit the possible actions they could do. Right now Amazon is well behaving and its supported by the customers. However, if Amazon were to change its actions, we would have a near monopoly that is not positively affecting people. Therefore, the theory of the author is to limit the power of Amazon by entroducing a competitor that would challenge the dominant position instead of waiting for the negative outcome.

maybe the article didn't lay out a case but that question should really be viewed from the perspective of more vibrant competition in the marketplace. We know in general that competition is good both for consumers and in the case of the technology sector competition is good for innovation.

I think the question applies to the likes of facebook (find me someone who is truly happy with facebook as a service, the whole world's social media needs cannot possibly be well served by the ideas of a handful of people in California), google..etc.

There is also the question of economic equity (geographic for instance), there is a direct relationship between the concentration of market share and concentration of wealth.