Plan 9 was built with the observation that high-resolution, bitmapped graphics displays were ubiquitous, and there was little motivation to keep the dated "tty" model as a basis for interaction. So you're expected to a graphical interface for working with the system, but it doesn't _need_ to be `rio`.
I wrote a bit about this a few years ago: http://pub.gajendra.net/2016/05/plan9part1 (I guess I should get around to actually supporting HTTPS here....)
My impression is that Plan 9 was a child of the technical workstation age - when we thought that the endgame was having ever more powerful desktops who did most things locally and connected over a network to send and receive files. In the end, our desktops and laptops have been fast enough for the past decade or so, while servers are becoming increasingly more like the mainframes we thought had their days counted and a lot of the heavy lifting is done remotely. A lot of what I do includes firing up a surreally large cloud machine, run a couple data transformations, and then unceremoniously delete that big workstation (that's actually a small slice of a humongous server). And, the rest, is mostly applications running on what I assume are clusters of cloud machines that expose an HTTP interface to my browser (a lot like the beloved 3278 terminal, but not nearly as clicky and tactile).