The open-sourcing of Java could be considered to be a counterexample to the picture you're painting. In spite of being opened up years ago, Java has not been me

The open-sourcing of Java could be considered to be a counterexample to the picture you're painting. In spite of being opened up years ago, Java has not been meaningfully fragmented as a software platform/development target. Of course, there are differences, such as Java being a language, library, and tooling, whereas Windows is an OS in addition to APIs and tooling. I'm sure you're right that there would be costs as well as benefits to open-sourcing of Windows.