That is, that you had a network transparent message passing system with high reliability is the key. That it was implemented at the OS level seems a side detail.
That is, if you were to want to build something like that today, I would remove "at the OS level" from the primary constraints in building this solution. Granted... depending on the rest of the requirements, it is highly possible one might go sans OS and just have an FPGA solution. Odds are, though, that would be overkill and more restrictive in possible solutions than necessary.
This is precisely my thought on microkernels. At face value, what is there not to like? In practice, the idea seems overly restrictive and makes it very hard to do some things that you sometimes wish to do.
This was not a theoretical exercise, it built a very deep and wide defensible moat, every Monday was a day pushing a record number of messages through the system since its inception, in other words, growth was continuous and downtime unbelievably (by todays standards) low. Which had a lot to do with the intrinsics of the OS.
And you have my word that without QnX as the cornerstone of that development it would have simply never happened. You can believe me, you don't have to but you're asking me for my reasons and then you proceed to dismiss them on the basis of your theories.
Practice > Theory any day.
If that is your argument, then it seems a moot subject. We'll never know. If Twitter had originated on a microkernel based operating system, would it have gotten the traction it did? Beats me. Not even sure how you could approach such a question.
Now, I fully agree with the view that historically a major way to get these was to go with a solution like QNX. My point is merely that the reliability of other solutions is not as far behind as they used to be. To the point that limiting yourself to only run on one of these systems seems overly restrictive. (And again, that is in today's world. Not the one of 20+ years ago. Probably not even the world of 5+ years ago.)