I don't follow this line of reasoning at all. By all accounts, Microsoft should have been broken up. They were convicted of abusing their monopoly position in a court of law. It was a travesty of justice that it was not carried out. Had this happened, the new companies would have been vastly more competitive, and the current state of computing might have been much better.

It's hazardous at best to assume the state of something with so much variables.

As for continuing being GAFA advocate (while still thinking they should be compelled to open up "Stores" to competition), vertical integration provides enough economy of scale to achieve ever greater complex things. Also, 1) you can't just "break-up" any GAFA. They're just gonna move out of State and be welcomed somewhere else, and 2) what you're asking put Government in a very awkward position as setting up both enough regulations to block new players, but also blocking player to achieve a certain size. Finally, this argument is getting awfully close to the "broken window" fallacy. You can't just "create competition" out of thin air.

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).