Market share depends on how the market is defined. Alan Reynolds has shown that the government narrowly defines the market to further its case. When all this is taken into account, Reynolds calculates that Microsoft's Windows was on no more than 70 percent of the PCs delivered last year. Another problem for the judge's monopoly theory is the ever-present potential competition. Entrepreneurs and venture capitalists are always looking for high returns.
Potential competition can soon become actual.
No firm is safe. A complacent, inefficient lone seller garnering high returns is begging for competition. As D.T. Armentano has pointed out, the most effective barrier to entry is low returns. The idea of potential competition makes a mockery of analyses that assay competition by counting firms or computing market shares. Believers in antitrust law don't understand this.