I actually don't think it was a monopoly. I think it created an opportunity for someone else to create better software. But instead we wanted to get rid of the so called monopoly and we secured Microsoft's position in the market. So if you want to talk about opportunity cost and state that Microsoft had a monopoly, then the opportunity cost is that we didn't let another better company take its place naturally and we spent a lot of tax payer money in politics to take care of a problem that didn't need intervention. I firmly believe we wouldn't be forced to use Microsoft today if we chose to do nothing.
What I mean is, we secured it by forcing them to create better products, so we made Microsoft more desirable for consumers. If someone dislikes Microsoft's monopoly and Bill Gates' fortune, then I'd argue intervention made it worse.