"It was a shitty period. There was a sense of futility to building software. You could build small software, but as soon as you reached a certain scale, particularly if you were a platform company, Microsoft would decide that they'd like to take your revenue, so they'd box your software out of their operating system with incompatibilities, launch their own competitor, and eat your lunch."
At one time, IBM controlled around 70% of the whole IT business, probably more. Mainframes, minis, comms, switchboards, typewriters. IBM even ran its own bank (IBM Credit Corp).
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.