Sure, but Windows had (and still has) a monopoly on desktop operating systems. Being a monopoly doesn't mean you have 100% market share, but rather that you're (a) the dominant player, and (b) your market share is considered sufficiently high that it can be leveraged to modify the market forces themselves. When you're a monopoly, additional antitrust requirements apply to you that don't apply to the lesser players in that market.
In 2001 when the lawsuits were happening Windows (Desktop and Server) had 80% market share, Android has 88% now. And while you can switch to iOS it's prohibitive in that you have to buy dedicated hardware.
In 2001 if you wanted to switch off Windows you could buy BeOS, Linux, or OS2 and run it on the same computer that you had windows on. Back then I did just that, switching from Windows to BeOS when BeOS was made free, I did my university work on BeOS running on a Toshiba laptop that came with Windows.
Right now if you have an iPhone or Android phone you have very little option to switch operating system on your device.
Android doesn't have 88% of the US market, only the world market.[0]
Also, what got MS in trouble was their contracts with hardware vendors. Companies like Compaq were required to pay MS for a Windows license even on computers they sold without Windows installed, and that was an abuse of MS' power. If Google is charging companies for Android even for phone they sell without Android installed, that would be analogous.
[0] "While Android continues to grow its share of the global smartphone market, it is actually losing share in the U.S. Kantar’s data shows Android phones making up 55 percent of U.S. smartphone sales compared with more than 60 percent in the same period a year ago." http://www.recode.net/2017/1/11/14239038/iphone-apple-androi...
I'm pretty sure it wasn't 80% of the Desktop market. More likely 90%+ according to what I could find, with MacOS being in the 3-4 percent and Linux at less than 1%.
And yeah, sure, you could run OS/2 or even BeOS, but these never reached statistically significant penetration.
Just as today you can run Sailfish, or Cyanogen/Lineage, etc... the latter supposedly has a market share of 3% or so. Better than BeOS, Linux, etc... in 2001.
