IMHO the problem is how monolithic the whole OS is built. How the hell is it possible that they "build Windows"? That includes things as diverse as file system

IMHO the problem is how monolithic the whole OS is built. How the hell is it possible that they "build Windows"? That includes things as diverse as file system drivers or notepad. If I want to make a small change to notepad, I should not have to check out the whole Windows source code, but just notepad. Then you could have different development methodologies for different projects. They should rip everything out piece by piece, and make it an addon. Then you'd have a minimal Windows core, a Windows shell, and the individual accessories. The shell alone would still be a huge project that you'd want to split up ideally. The challenge with such a model is that you have to be really strict with interfaces between components. You have to define these interfaces, you have to quickly provide other groups with interfaces they need, and you have to provide backwards compatibility as long as any other component uses your interface. It's much more work, but it's better than the ad-hoc coupling you get if everything is one monolithic build.