> What killed it is they, for some reason, spent time and treasure port XWindows to it instead of working on a TCP/IP stack

Just to be clear, there's a whole heaping helping of really big work required to get there either way, since Coherent was still essentially a 16-bit OS that started out as a V6 clone on the PDP-11 and hadn't really kept up during the 1980's with any of what was happening in the wider UNIX world beyond, oh, the Seventh Edition.

The key starting point for Coherent 4.x was essentially a single piece of work (almost all of it provided in a single .c file) that let a 3.x kernel boot into 386 protected mode and provided a system call handler that more-or-less emulated the call sequence specified in the Intel iBCS specification and mapped the iBCS system calls to the original 16-bit Coherent 3.x ones, and a very basic loader for statically linked SVR3 COFF files.

[ See https://en.wikipedia.org/wiki/Intel_Binary_Compatibility_Sta... which essentially formalized a subset of what the SCO port of System V did. Note, that omitted a number of system calls; I'm fairly certain that the iBCS manual didn't include any of the socket-related system calls anyway, so the public spec for what people took to mean "will run SCO binaries" left a lot to be desired. ]

Everything else around that was still, essentially, 16-bit code that was written around some variation of Sixth Edition or Seventh Edition, not System III/ or System V (so to be iBCS compliant the bible was the System V Interface Definition, aka SVID), let alone any of the things that were being added to POSIX.1 or POSIX.2

So, the work required for releasing the V4.x series of Coherent included a massive number of upgrades of almost everything, along with all kinds of awkward technical compromises to a) not break any existing things from the 16-bit userland, which stayed 100% supported, b) navigate between SVID and POSIX because while we on the one hand wanted to be able to run SCO binaries (hence, do things per the SVID) while we had all kinds of enthusiasts really wanting to use the ability to 32-bit code to run various shar archives of GNU project code unmodified.

So, I'd just note about that 16-bit userland from Coherent 3.x was that virtually everything on the (nominally 32-bit) version 4.x install floppies - all the standard utilities, pretty much everything that could be - still was built as 16-bit a.out binaries to keep the install size down. Pretty much everything we did in terms of C libraries and such had to be, and had to stay, buildable both 32-bit and 16-bit from the same source since both subsystems were needed. At some point we could conceivably have (at a technical level) cut over to all-32-bit, but just from the point of view of media duplication, doing that and still staying at a $99 price point wouldn't fly, and also the resulting increased memory footprint wouldn't have made existing customers happen since the kernel didn't have any support for demand paging, at a time when lots of people had just 1-2Mb (or 4Mb for a relatively beefy system) of RAM.

[ So for all the work required to let /bin/sh able to run successfully run multi-megabyte autotools scripts that tried relying on POSIX.1 shell features? Yup, everything involved in POSIX.2 support was all done with code that stayed 16-bit. ]

Without getting too much into the weeds of all the other various challenges involved getting a system like this out, for TCP/IP specifically the question was what kind of TCP/IP stack to pursue and most critically of all the question of where were the NIC drivers going to come from. There was not going to be any plausible way we could work with third-party SCO driver binaries, which were the most common type of UNIX driver that NIC vendors did provide at the time; SVR4.2, however, had STREAMS and the DDI/DDK so there was a plausible, reasonably portable driver model which seemed like a realistic future, but just licensing Lachmann and trying to bolt that into the kernel wasn't realistic at the $99 price point (and at the time, there weren't much of any third-party NIC drivers either).

To make 4.x happen we had to come from a very very long way behind, and getting to where we needed to go while staying within the constraints of a small staff and that $99 price point just made for an incredibly tight set of constraints. None of the resulting tradeoffs were fun to make. We did our best.