So I'm going to answer this question a little differently. I think it makes sense to look at Apple, Google, and Microsoft by their primary business model. Apple primarily makes money by selling devices. They get you to buy more of Apple devices by making Apple services integrate easier on Apple devices. And of the money they make from devices, the vast majority comes from the iPhone. Google primarily makes money from advertising. Their goal is to get your eyeballs looking at a screen through Google stuff. Gmail, chrome, and android are the biggest in capturing your activity and leveraging this in advertising. Google can make so much money this way, android can be made open source and given away without licensing fees. Microsoft, on the other hand, makes money primary from licensing fees through cloud services, server products, Office, Windows OEMs, etc. They have successfully monetize software over the decades. Google essentially decided monetizing software is not worth it and to just depend on advertising.
What I see as a potential threat to Microsoft is their dominance in the PC operating system. Android has a small chance of becoming a desktop operating system. So I guess I'm saying Google is the biggest threat to Microsoft. There are so many people using android phones, I can see Google making a desktop version of android, and people would jump on it. The advertising and open source business model worked great for android phones. I could see that continuing on to the desktop. I know they have ChromeOS for desktop, but ChromeOS is so limited, I can't see it gaining too much market share. Making a unified Android for desktop and phone would be a big threat. I thought Google was doing this in a project called Andromeda. I guess it was canceled. But that doesn't mean they will not try again.