Most users never buy Windows. They buy a computer. Most computers are never upgraded, they are just demoted and replaced. The second-hand user simply continues with whatever OS was originally installed.
DELL sells about 150,000 computers a day, 365 days a year. Considering their market share, that means about 150 million new computers are sold every year, and 90 percent will be delivered with Windows.
Windows will add about five percent to the price of the computer.
Now it's around 30%. While the price of everything related to computers has come down significantly since 1990, the price of a Microsoft operating system is just about the same today as it was then.
But monopolies don't skew the market or anything.