Windows 9? What does a web serach have to say about that? Oh, there are a few interesting things. Indeed, skipping Windows 9 and going to Windows 10 was a nod toward the fact that Microsoft was really trying to turn away from the terribleness of Windows 8, similar to what Windows 7 did to Windows Vista.
Also, there is this other interesting article about the user interface, and some questions on innovation, short term versus long term gain.
20190606/Bing windows 9
20190606/https://www.extremetech.com/computing/191279-why-is-it-called-windows-10-not-windows-9
20190606/https://www.extremetech.com/computing/191255-windows-10-familiarity-breeds-contempt