Traditional economics holds that monopolies are bad for consumers. Essentially, this is because the monopolist can do whatever it wants, and the consumers have no alternative. The monopolist has no incentive to keep prices low, to innovate, or to improve its service level, because the consumer can't exactly go elsewhere.

These days, however, the threat of new entrants into any given market is much higher. Starting costs are drastically lower than they were when conventional microeconomic theory was being worked out. All things being equal, as a consumer, you still don't want a monopolist owning the market -- but the monopolist has to be somewhat cautious, for fear of disruption.

It seems highly unlikely that Amazon will ever operate a true monopoly over all categories of retail. Nor will the government (and competitive lobbying) allow it to get there. But an oligopoly is certainly possible.

(On a micro level, Walmart has basically functioned as a de facto monopolist in certain regions where it has virtually no competition. Those days are over, though.)

Monopoly profits, from the very beginning of capitalism, have always been necessary for significant investment. Micro textbooks are just wrong. For most businesses the supply curve slopes down, not up.