If they are "doubling down on open source", maybe they could get open source Darwin working again? From what I've heard, while they still release some of the source code, it is no longer in a form in which it can easily be compiled into a usable operating system – e.g. some of the components they release source for can't even be built because they rely on unreleased Apple private header files – basically, things have gone backwards on this front compared to earlier versions of OS X.
(From what I've heard, I don't know how true it is, Apple became less open with Darwin, around the time of the PPC-Intel transition, out of fear it was making life easier for people trying to use OS X on non-Apple hardware easier.)
I can think of one use. Suppose I am developing something non-GUI (e.g. a database, an application server, a compiler or interpreter for some programming language, etc.) While macOS is unlikely to be the production deployment platform, getting it running on macOS will make some developers happier (yes, they can always use a VM or Docker to run the Linux version, but running a native macOS version has its advantages.)
So, now I have a non-GUI app for (among other platforms) macOS, I want a build machine for compilation and automated tests. Ideally just a VM somewhere (maybe even in the cloud). To legally run macOS in such a way, it needs to run on Apple hardware, which adds expense and complexity. However, I could run a Darwin VM legally on any hardware. Now of course, Darwin isn't quite macOS, but for a non-GUI app it might be close enough (depending on exactly which APIs it uses and whether those APIs fall into the open source Darwin subset).
Is that a particularly common use case? Probably not. But there probably are multiple open source projects and maybe even some proprietary software packages for which that use case would exist.
I'm not sure if the problem is Apple or a lack of interest and community.
I'd definitely be up to trying to build it sometime in the future. I'm a little busy at the moment. I'd expect it to take a few days at least based on my experience with Linux from scratch.
(From what I've heard, I don't know how true it is, Apple became less open with Darwin, around the time of the PPC-Intel transition, out of fear it was making life easier for people trying to use OS X on non-Apple hardware easier.)