If people were selling copies on the street for $2, that suggests that $2 was roughly the right price point (in that area) to maximize profit, and Microsoft couldn't have made more money by selling it for more.
That argument ceases to hold in the case of BitTorrent, where the distributors aren't trying to maximize their profit (or make any at all).
See previous response, Microsoft actually did change their pricing and captured a huge chunk of value. It was significantly higher than $2/CD. When selling information there is a tail, a few people will pay whatever you ask, a few more will pay less than that, Etc, until you get to all of the people who would obtain a copy if it were zero cost. That defines the "value curve" of the information. It is not a straight line. Because it is "hard" to justify large differences in price for the same product, one of two strategies are used a "steadily decreasing price" or a "fixed but relatively low price". Depending on the product different things make more sense. Games for example benefit from the 'expensive at first but cheaper later' model, reference works benefit by capturing as much market as possible quickly.
That argument ceases to hold in the case of BitTorrent, where the distributors aren't trying to maximize their profit (or make any at all).