SIP is not some irrelevant detail, nor is it an obviously good "modern security practice". A system that the hardware owner cannot modify is not really on the same wavelength as traditional UNIX.
Traditional UNIX is does not mean having an open source core you're intended to tinker with. Traditional UNIX was entirely proprietary, connected with well-defined text-based interfaces. This is entirely orthogonal to user control and manipulation of the OS.
AT&T was forbidden by its consent decree from productizing Unix. Licensees received the source code, on an as-is basis, with no official support from AT&T.
As a non corporation end user with a sublicense from one of the above licensees (which is the apt analogy for an individual user in my opinion) you most certainly did not receive the source or have the rights to it.
You then likely hacked the source code to make it work on your machine.
I can understand that others might include the commercialization of the 1980s and the Unix wars as part of "traditional Unix", but even then, my IRIX system ca. 1990 supported end-user kernel extensions, to handle new types of devices. Our imaging hardware was not "connected with well-defined text-based interfaces".
It doesn't mean end users had access to the source code.
In fact, vendors would have been prohibited from distributing AT&T source code to customers who weren't themselves AT&T licensees, just as CSRG was prohibited from distributing BSD to customers who weren't licensees before 4.4BSD-Lite.
The same goes for most other proprietary source-available products. For example, game developers can't open source Unreal Engine projects even though Unreal Engine source licenses are available to end users for free.
At a bare minimum, you would almost have to have source to the vast majority of the kernel because there was no kernel modules. Sysgenning for a particular system involved compiling the kernel for that set of hardware.
I'm literally working on an early sun system emulator right now, heavily assisted by the standard sec tape that came with the standard software distribution of sunos, which has plenty of AT&T code.
As jeremyjh wrote at https://news.ycombinator.com/item?id=50032733 , "A system that the hardware owner cannot modify is not really on the same wavelength as traditional UNIX.". Since traditional Unix was source-available, with the expectation that people would hack on the source, I fully agree with jeremyjh's characterization.
I think you are talking about a later era in Unix history? The early Unix licensees were educational institutions and commercial sites, not vendors or resellers.
Every one of those academic and commercial end users got the source code.
From https://en.wikipedia.org/wiki/History_of_Unix :
> "In 1973, AT&T released Version 5 Unix and licensed it to educational institutions, and licensed 1975's Version 6 to companies for the first time, ... Anyone could purchase a license, but the terms were very restrictive; licensees only received the source code, on an as-is basis. ... Unix was entirely unsupported by AT&T"
From the same link, "Perkin-Elmer began distributing Wollongong Unix for Interdata as Perkin-Elmer Edition 7, the first version supported by a computer company". That was in 1980.
So while yes, "Traditional UNIX was entirely proprietary" and could not be redistributed to non-licensees, it was also provided with "full user control and manipulation of the OS", and not merely "connected with well-defined text-based interfaces", quoting jdiff at https://news.ycombinator.com/item?id=50033080.
SIP is as much defense from the likes of Adobe and Google as it is from more traditional malware.
I think that piece by piece Apple wants to go into direction of making macOS just a desktop version of iOS, that is closed and impossible to modify. They will use of course security as an excuse, the practical thing is that with iOS they succeeded because they went to the market with an OS that was already closed, but making users accept that in new version downloading a software from the internet and installing it it's no longer possible takes time. But they are getting there soon to me.
The reason why that won't happen (but more and more warning prompts all the time because of this modern world...) is because macOS is a self-hosted platform.
Apple _needs_ macOS as an unlocked OS at the very least as a development platform to develop their operating systems on. Not having the self-hosting requirements allows to tighten the security model a lot.
Notably, with a Unix terminal, a huge problem for sandboxing is how to determine permissions granularly, whereas a UI flow has significantly better answers to that.