We're not discussing whether the source code for XNU/Darwin/Mach/MacOS itself ought to be open, which is another question entirely. (Obviously everyone would sooner it be open than not open, in a first-order analysis, but, as with medical patents and the like, a proper analysis would have to ask whether it would even have existed if its source code were to be open.)
[0] Or rather a non-moot point, for the pedants.
The overwhelming majority of XNU/Mach was created as open source. It is quite unlikely that macOS would exist today if those components had not been created under an open source license.
I was replying to this remark:
> Most users – hell, most developers – do not give a toss about having the source code for software that's shipped with MacOS.
I've been developing FLOSS for several decades now, but I rarely compile stuff from source anymore (except my own projects). However, the availability of the source code for the tools I use is very important to me, even if I never exercise it myself.
In addition, general access to the source code is important as a learning tool for programmers, if only "to see how it's done". You may never build such code, but being able to take a look at it can be an important foundation in the continuing education of software developers.