The problem is that “Darwin” really means “whatever random subset of projects Apple decided to open source”. Perhaps once it was more of a cohesive thing; I know there were attempts by others to build a Darwin-only operating system, though I wasn’t around then. But even then it was never anything more than a toy.
In any case, today Darwin really is just a subset of projects, which can’t be used (or sometimes, can’t even be compiled) without other projects that are closed source. The projects that are open source are still updated, but new projects are rarely open sourced. And that includes rewrites: when a project goes through the rewrite treadmill and gets replaced with a shiny new codebase written from scratch, that new codebase is often placed in a new project, which like any other new project is usually not open source. Thus, even though open source projects usually aren’t directly changed to closed source, the amount of functionality provided by open source projects shrinks.
In any case, today Darwin really is just a subset of projects, which can’t be used (or sometimes, can’t even be compiled) without other projects that are closed source. The projects that are open source are still updated, but new projects are rarely open sourced. And that includes rewrites: when a project goes through the rewrite treadmill and gets replaced with a shiny new codebase written from scratch, that new codebase is often placed in a new project, which like any other new project is usually not open source. Thus, even though open source projects usually aren’t directly changed to closed source, the amount of functionality provided by open source projects shrinks.