I wouldn’t call Microsoft’s success in the 90s unlikely. They had a decent product at a low price for commodity hardware when nothing else was as good for as cheap. They also had decent marketing. The company worked hard and delivered. That’s not unlikely, it’s good execution. The unlikely part was something more like OSX taking Microsoft developer market monopoly away.
You needed Visual Studio and C++ to automate even the most mundane things in Windows. Without decent scripting or command line Windows was a developer's nightmare compared to Linux.
Anything providing something like Linux with a polished surface and support for the tools of the rest of office IT (e.g. MS Word) was going to blow Windows away in this area.
OS X was the product of NeXT and Jobs returning to Apple, and those two weren't the only option. There was also Be and Gassee. Apple had to have compatibility layers for that transition, and the amount of work involved was far higher, making success less likely.
At this point, MS had been among the largest purveyors of developer tooling in the PC space, MS-DOS and Windows 3 were pervasive enough to have emulators on workstations, and Apple was nearly dead. When it came to microcomputers that were not workstations (so almost all by percentage), MS was king. They were pushing, for the time, a decent product at a decent price.
Apple coming out of nowhere with a solid product just wasn't likely. It would have been far more likely for the company to die. They did well, executed well, marketed well, but it wasn't likely. It was very risky. Facing down Microsoft at that time was Herculean.
