@Rzaj having a monopoly means that you're literally the
only one able to provide a "service".
The simple fact that alternatives exist, means that Microsoft cannot be a monopoly, regardless of the market share they enjoy. Alternatives existed since the birth of Microsoft. Apple came out with their first computer nearly 10 years before the first windows PC. Apple were also the first to release a graphic OS.
If apple were smart and allowed others to build computers and install MacOS on it, they'd be way more popular. (instead they preferred to be seen as the "premium" brand... You get what they offer you and that's that)
When you, as a consumer or the CEO of a company, have the option between a 500$ "good enough" machine for word processing or a 1500$ "overpriced" Apple PC that you wouldn't even use to its full potential anyway.... The choice is easy to make for a lot of people, even more so in the enterprise world.