The paper actually illustrates many advances in OS abstractions and techniques before and after UNIX that was superior to them. There's others outside this paper as well. Many, even on limited systems, smashed it in terms of IPC performance, fault-isolation (esp drivers), concurrency safety + throughput, shared memory support, management, and so on.

It was discovered far back that there were limits to how reliable or secure the UNIX API could get when improvements were attempted. Look up UCLA Secure UNIX, LOCK/ix, Trusted Xenix, Mach ports, L4Linux ports, and so on. Outside of isolating a whole OS (i.e. virtualization), they had to change the behavior of and/or eliminate many calls because they had too many negative effects on system state in fault or malicious conditions. This was not true of many alternative architectures that naturally decomposed systems, isolated faults, protected at interfaces, and/or implemented language-based protections. It's just a UNIX + C thing.

Let's re-illustrate this economically. MINIX 3 took a few people a few years to get it to a stage where it was quite reliable ("self-healing") as a console OS and easily modified due to architectural choices. How long did it take UNIX/Linux kernels to do that? No, really, because Sun's and Linux's kernels were still crashing on me in the late 1990's plus corrupting my filesystems on occasion. I'm wondering if it took two decades with hundreds of contributors or three decades? Either way, I think the effort vs results says something about their architecture and implementation strategy.

Similarly, Oberon was thrown together in a short time with few people while achieving more reliability, consistency, language-oriented protection, and concurrency performance (with AOS vs forking etc).

Many examples of systems doing it better with better results across the board. That also directly contradict UNIX model. It never won for superiority: just simple enough to run on the "cheap" hardware back then, open-source, and lots of users already.