What one wants is Plan 9 as a
model for what may be a family of hardware APIs. It makes sense to
promote massive parallelism, but the API to it should be sufficiently
simple for a single individual to manage.

This is the what I wonder about. Is this possible at the hardware level and still support an equally simple, understandable, yet capable, software system on top? By extension, would Plan 9 would run on such a system or if it would require some fundamental changes to adapt to it. For example, does C really need to be thrown out or can it be revised.

Most computing devices today are single-user, even those like my new
Android phone that offer shared user capabilities. Incidentally, the
authorisation model in this case is inadequate for my purpose (share
with a pre-teen).

I am in this boat too, however I have a general aversion to cloud computing and so I would need some household multi-user systems for data storage and heavy processing tasks or some distributed equivalent.

So we have layers and we need the complexity to be shoved into
well-tested, sealed boxes that can be trusted, while the surface
remains as simple as 9P.

My trouble is that I don't trust the sealed boxes anymore after Meltdown, Spectre, Rowhammer, etc. Perhaps simple and auditable hardware might help with this.