[chumali]

This is an arbitrary definition that few economists would accept. Market concentration exists on a spectrum ranging from perfect competition to full scale monopoly. The determinant is the ease of entry and exit and the extent to which the outcome diverges from utility maximising point.

In the case of Amazon there are sufficient barriers to entry that if Amazon were to make profit maximisation its goal, then it could increase prices and enjoy above market returns for a significant period even in spite of the eventual 'possibility' of competition.

To suggest that regulators should wait until the market is fully captured before considering a firm a monopoly is patently absurd since by then the efficiency losses would have already been realised and the remedy becomes harder to implement.

Also, I urge you to find anyone who agrees that there has been an increase in the quality of products available on Amazon. On the contrary, there are countless stories of people lamenting the decline in product quality.