Honest question, not trying to be dismissive: This architecture sounds old to me, as in things were built like that in the 80s or earlier but evolved past. Is that so? If so, what makes those decisions newly relevant?
When this scheme was designed, it was frankly a little bit nutty. The CPU, auth, and file servers would be VAX or Sun systems, costing tens of thousands of dollars, and the terminals would be either slightly cheaper Suns or IBM PC-compatibles costing thousands of dollars themselves. Today, you could probably cobble together a passable network for a few hundred dollars, assuming you use cheap Atom boards for everything except the CPU server (which is meant to be a beefy compute box, but let's be honest nothing in Plan 9 uses a lot of cycles). This makes the architecture more sensible than ever.
There was a centralized computer which was programmed via punch cards which you could dial into to submit your specific simulation and later return to for results.
However, if you compare setting up a Plan 9 auth server to setting up a Kerberos server... well, basically anything to do with Kerberos makes me long for death. The Plan 9 auth system is one of the best things they made and I highly recommend checking out the paper: https://css.csail.mit.edu/6.858/2013/readings/plan9auth.pdf
But what machines are, in practice, multiuser today in that way? My work computer has only my account, and sometimes a temporary one if a technician needs to log in to troubleshoot something. For home use I don't think it makes sense. And for a prod network, again users aren't logging into the machines directly.
"those who believed in Harvey were thought to be insane, but eventually found not to be."
So they recognize that many will think it is an insane idea but are playing a long game to prove that to be incorrect.
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About the Name
So, why “Harvey”? Mostly because we liked it, but there are at least 3 other good reasons based on the origins of Plan 9 operating system:
Harvey is a movie, Plan 9 is a movie too. Harvey is a rabbit, Glenda is a bunny. Most importantly: in the movie, those who believed in Harvey were thought to be insane, but eventually found not to be.
Actually most modern OSes in use are even older in their concepts.
Plan 9's concepts as described above have slowly creeped into Linux but not fully. So the architecture above was only experimental in the 80s/90s and is still nowhere available in the mainstream today.
So your concern is like saying "we've had macros and closures since the 70s" in a world that still uses Java/C#/etc.
It is not clear why such things were never adopted. There were many licensing problems on the way, and a Microsoft monopoly pushing things into even older architectures than the competition. There were also failed modern projects, but there is little evidence to decide if it is a hard problem, greed people forced everybody into a worse path, or if it is something people do not want.
Plan9 "fixes" many of the flaws/deficiencies of the original UNIX, such as that one.
Whats cool about this Plan9 project compared to the original Plan9 is one does not need to use the Plan9 compiler to compile it. One can use gcc, clang or icc.