The possible reading of the dMGs that treats aspects of market structure as inherently harmful or as conclusively identifying competitive harm raises more than one
difficulty. One problem is that merger enforcement would not succeed in stemming a trend toward concentration when that trend is the product of economic factors like
growing scale economies, increasingly powerful network effects, or endogenous sunk costs—or if it did succeed, it may well do so at the cost of substantial harm to
counterparties, including consumers and workers.9
Another problem is that market structure is not always a good indicator of market power. Market structure can play a valuable role in merger enforcement, as with
the structural presumption in horizontal merger analysis. However, other aspects of market structure relied upon in the dMGs—trends toward concentration or vertical
integration, and extensive vertical integration— are related to the exercise of market power from merger only under some circumstances or in conjunction with other factors. They are not related to market power with sufficient consistency to justify relying on
them the way the dMGs could be read to do: either as near-conclusive indicators of market power on their own or as intrinsically harmful features of market structure.