You're right, you can't just create competition out of thin air. The competition already existed at that time. Part of the abuse of their monopoly position was to use the dominance of Windows to also capture other markets, including the Office market, Web browsers, etc. We would have had a far healthier ecosystem if the monopoly had been broken up. Look at where Lotus SmartSuite, Corel Office, StarOffice, and all the rest are today. They were not able to compete commercially in a market distorted by a monopolist, and today are shadows of their former selves.
Just because there was competition doesn't mean it was any good, nor that it was gonna last. Microsoft "decline" largely came from new technology they laughed at (smartphone and cloud computing) and to a large extend, missed on. Such an evolution is typical from the high inertia inherent to such a large structure. It forced them to evolve, for the better. Same will likely happen over time with Google/Apple, their structure will get more and more rigid, they will stop providing as much benefit to the user, competition will appear and their will be forced to open up. Apple is already starting to lose ground on the self-repair front, and both are more and more called out on the rigidity of their stores.
Amazon/Facebook is a different problem. Amazon is colliding with the old retail chain and weakening their power in DC (Amazon is not doing anything more that has been done in brick and mortar stores ~forever). As for Facebook, it is threatening Governments themselves (privacy is just just a convenient red-herring).
