The hardware owner can modify it all they want, disable SIP. For a large swath of average and non-average users that will not be modifying the closed-source OS they're running, it is obviously good.
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.
Also worth keeping in mind that macOS is intended for use with commercial third party software that’s repeatedly been proven untrustworthy, doing things like prodding system internals and installing dodgy kernel extensions where neither is warranted.
SIP is as much defense from the likes of Adobe and Google as it is from more traditional malware.
For now SIP can be disabled (on mac, not of course on iOS that is completely closed) but I wonder for how much. It's clear that the desire of Apple is to make macOS a closed system like iOS, installing third party software outside the AppStore is more and more difficult, with Gatekeeper that asks you to enter the system settings and warn about "potential insecure stuff" each time you want to run an application downloaded from the internet not signed by Apple (yes I know that if you disable SIP you can get rid of Gatekeeper as well).
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.
Traditional UNIX was entirely proprietary, with source you're intended to tinker with.
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.
Sure, but that just means UNIX vendors had access to the source code.
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.
You would absolutely normally get source to the vast majority of the unix components with a unix install.
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.
Sure, if by “you” you mean universities, Sun, DEC, IBM, etc.
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.