Factually this is one of the weakest parts of the government's case. It will be difficult for Netscape, Sun, Oracle, etc. to prove that MS's conduct foreclosed their ability to make a successful OS, absent evidence that any of these companies were close to succeeding. Such evidence, however, was not presented during the government's trial. Even with proof that there may have been a rival OS, it could still be a formidable task to show that entry would have caused MS to lower its price. The second category of anticompetitive effect concerns MS destroying competition in the browser market.
Browsers are now a free product.
Prior to MS's allegedly predatory campaign, however, Netscape had been able to charge for its browser. This price surely would have had to come down if Microsoft had entered using legitimate tactics, but perhaps not to zero and perhaps not as quickly. Finally, there were a variety of other anticompetitive actions that Judge Jackson found helped MS maintain its monopoly position in the OS market. The government just proved that they exist. Some quantification would be required in estimating damages from each act.