Many enterprise projects usually runs multi-year, its nearly impossible to make a code base that can be re-written in a month.

How would that work for say the windows code base?

I have no OS dev experience, but I like the idea of many smaller specialized programs that are able to communicate with each other. Another problem with large and slow moving code bases is that things around changes, like hardware getting better/faster so software architecture that made sense 30 years ago might not be the best solution today. An OS should probably not be developed by a single team or company, there should be an open core/kernel, then companies could instead compete and specialize in different combinations and support of systems, apps and services developed independently, where each part could be replaced.