Standards
I think the article misunderstands the purpose of the Unix software standard that did succeed - POSIX.
POSIX came about largely because of the edict of the US DoD that it would not accept procurement bids from companies unless the design complied with open standards for both hardware and software. VME got chosen as the hardware standard (and is still alive, supported and functional today, though there is now also the far quicker OpenVPX too). POSIX became the software standard. The reason the DoD gave this edict is that, previously, it had been paying extremely large support costs for bespoke processing system for things like radar, communications, etc.
Whether or not the article is correct in saying that POSIX is "too general" to have succeeded by the article writer's terms, POSIX most certainly did succeed from the Department of Defence's point of view. The vast majority of technology-based systems across NATO are based on VME / OpenVPX, and POSIX. Software can be ported from generation to generation with minimal effort (compared to before POSIX). The price of development and support paid by the DoD for its very complex systems dropped very significantly.
And, if you can believe it, the risk in procurement has dropped. Essentially it is easy for equipment to at least pass the environmental testing it'll be subject to. The hardware manufacturers have become good at designing for the military environment. The DoD's, MoD's engineering standards have excellent data on what different environments are like in terms of temperature, shock / vibe, electrical supply, so it's been possible to make sure that the component parts survive. It doesn't mean the whole system works, but it should at least not fall to pieces!
If I were to guess, the problem being faced by a lot of these military systems is the slow demise of Xorg. There's quite a few military system based on XServer.
Windows!
Surprisingly, the open standards hardware that the DoD mandated opened the window for Windows to play a part. The hardware manufactures that glued down Intel chips into VME cards, or OpenVPX cards, essentially chose to make them PC compatible. So it became possible to install Windows. There are a fair few systems based on Windows, largely because of the availability of developer resources and Microsoft (by then) having a well deserved reputation for backward compatibility.
The irony now of course is that Windows itself is now an excellent platform on which to run POSIX compliant software, in WSL. WSL is interesting because you can in principal run an old / out of date Linux plus software combo, with security handled by fully-patched Windows. Like it or not, Linux has become one of the key POSIX platforms in military systems, but is now being dragged in all sorts of unhelpful directions by RedHat (systemd, gnome, etc), and increasingly the best option for long-lived Linux software systems that do not want to upgrade every 3 months is to run inside WSL.
Future Direction, Unintended Consequences of RedHat's Trajectory
DoD still mandates POSIX, and increasingly Linux isn't POSIX compliant (thanks to SystemD).
For example, for decades C code does name resolution by making a few library calls, and these library calls are the same on Linux, *BSD, Unix, VxWorks, INTEGRITY and other militarily significant operating systems. SystemD has introduced an alternative that involves making a request via dBus. Now, for the moment, SystemD has not displaced those well understood library functions, the dBus route for name resolution is an option. But, for how much longer? They're already re-routing conventional library call DNS requests to resolveD by messing with the default configuration files.
Given the attitude of RedHat / IBM, and their SystemD / Gnome teams, I would not put it past them to deprecate the library calls, and use their weight within the Linux distro world to make that stick.
If SystemD does start gutting Linux's compliance with POSIX at the software API level, this will cause military equipment / system providers a bit of a problem; they really cannot go that way. So there could be some very monied companies looking for a Linux alternative, with the motivation to put money into it. FreeBSD strikes me as a very strong candidate going forward.
This could hurt Linux badly as there might be strong demand for things like FreeBSD instances on AWS. Someone has already tried that I gather. And if there is plentiful supply of non-Linux based resources out there in the world, there may be others keen to get away from systemD. Certainly, with RedHat's current messing around with licenses causing no end of anguish, one has to consider the consequences of RedHat's grip on things like SystemD / Gnome. If RedHat were to buy Ubuntu (not impossible), possibly that'd be Linux in effect becoming owned by RedHat. There may still be a Linux kernel project, but if RedHat has bought Ubuntu then there'd not be many distros out of their control, and the only Linux kernel anyone is running comes from RedHat.
If RedHat are motivated to lock software and users in to their version of Linux (their corrupted version of POSIX), it won't be just a few OSS enthusiasts unhappy about that. It will be the military-industrial complex too and, indirectly, Uncle Sam.