To maintain this monopoly Microsoft employed tactics like offering discounts if OEMs promised exclusivity. Basically punishing any manufacturer that might want to ship another operating system. My read on the browser verdict was that this was what the justice department thought was sufficiently low-hanging fruit to convict Microsoft. But it was far from the only anticompetitive tactic Microsoft used at the time. "DOS ain't done till Lotus won't run"[2]
[1] I'm ignoring exotic stuff like SGI workstations that were priced out of reach of typical consumers.
[2] https://news.ycombinator.com/item?id=10434133
Edit: Here's an example. In 2002 Dell started offering PCs without Windows pre-installed[2] - this was considered a big deal in the linux forums I hung out in. Why? Because until then you had to pay Microsoft to own a PC (practically the only consumer computing hardware available at the time), even if you wanted to install Linux on it. This is like if nearly every phone had to come with iOS pre-installed (and Apple collected a licensing cut), even if you wanted to install/use Android.
[3] https://www.computerworld.com/article/2577666/dell-offering-...