Microsoft happened to steal a very small part of IBM's monopoly, which got bigger over time, but IBM's revenues are still bigger than Microsoft's even 32 years later. (And IBM has been spinning off or selling whole businesses along the way, eg the PC business to Lenovo; spinning off Lexmark).
I was talking to one US corporate IT manager who said IBM was still a third of his budget cf 3% for Microsoft.
Big picture: Microsoft's monopoly has always been much smaller and less powerful than the IBM monopoly used to be. However, in time, all tech monopolies tend to get largely undone by new technologies.