In 2014 Apple Announced they had around 80M Active Mac Users.
In 2017 They had over 90M Active Mac Users.
In 2018 They Reached 100M Active Mac Users.
Apple has a fairly stable Mac business, and ships 18 - 20 Million unit over the past few years. They are also very proud of their consistent ~50% of Mac sold are first time Mac user.
If no one was leaving the Mac ecosystem, you would have expected 9 to 10 million increase of active user per year. Except it didn't happen. The growth rate has slow down dramatically. They are gaining ~5M user per year.
There is also another stats Apple gives out, nearly All of the New Mac user were from China. And that is not surprising given the whole iOS ecosystem and games development requires Mac along with the rise of middle class. But this also tells you most of the New Mac User are from China.
This hypothesis doesn't takes into account how Apple measures its "Active Mac Users", but suggest Apple is losing millions of Active user outside China. And since US has the largest Mac user base, I would not be surprised if it lost ground to Windows was an observation from a US users perspective.