"Agreed, but what made it into mainstream computing we are aware of and touch and fiddle with?"

Many things made it into mainstream. Use of high-level languages with GC and automated checks. Mainframe model and many techs reinvented in cloud market and Xeon CPU's. Clustering is mainstream. Microkernel and hardware-up approaches dominate mobile research and products. Highly concurrent desktops like BeOS anticipated. I'm still waking up but this is what I remember off top of head.

"I don't consider a deeply embedded baseband processor to be mainstream outside the handful of radio engineers.""

Damn, my sleepy ass might have read it wrong. Still might have been right to cite it given things like Samsung Oxynos, big cores and little cores, are getting more prevalent. Low RISC has main core plus tiny cores. Even embedded are mixing cores of different strengths. What it gets you is significant boost to concurrency with lower power and cost. On other end, you can emulate Channel I/O from mainframes to get great throughput:

https://en.wikipedia.org/wiki/Channel_I/O

"It's crazy how Burroughs (B-5000) solved many issues elegantly and Intel's team with i960 lost the internal challenge to the x86 group. It may have been before its time to some extent, but worse isn't better when it concerns the underpinnings of computing."

My thoughts exactly. Burroughs is being reinvented under crash-safe.org with functional programming on top. Draper is integrating the HW enforcer with RISC-V with open-source plans. Some hope. i960 was clever. Lessons learned is to ensure compatibility with language or whatever is popular to piggyback on popularity. CHERI is doing that. Otherwise, you might die off.

"but the only mainstream microkernel'ish remnant I could point to"

Microsoft is taking the lead integrating cutting-edge tech like driver mitigations. Unfortunately, mainstream desktops, etc mostly ignore this. However, in embedded, there's lots of microkernels in use. One mainstream product, Blackberry OS & Playbook, used one of best ones called QNX Neutrino. Speed, responsiveness, and reliability are excellent. Too bad on network switches as Linux isn't going to beat QNX on reliability. The reason might be that they're usually deployed in HA configuration that survives crashes anyway. Long as not byanzatine failure, that works well enough for corporate environments.

"The success of UNIX in the 80s led to the unsatisfactory OS architectures we're using today. It also led to the rise of C and the introduction of trivial attack vectors all over the place. Even plain Pascal was much safer and used on Apple systems above ASM."

The success of System/360 + COBOL, CP/M + DOS + x86, UNIX + C, and recently Mach + UNIX + Objective-C. It was a team effort. :) Far as Pascal, you might find design and assurance sections of the kernel below interesting. It was written in Pascal for safety and first secure kernel.

http://aesec.com/eval/NCSC-FER-94-008.pdf

Btw, if you want, I can try to dig out an interview with Dr Schell that I found that traces his invention of INFOSEC field from the start. It's a long interview but was very worth it. Many surprises along the way like corporate types wanting high assurance but IT industry fighting it. And Burrough's legacy going further than you thing... even into Intel x86 CPU's. Not everyone wants to spend 30 min reading an interview though so I understand if not.