I'm not exactly making a value judgement here. I like Plan 9, but not enough to use it daily, in part because I like to do things like leave comments on HN and do flashcards on memrise. I'm just pointing out that the capacity of computers isn't the only thing that changed since this golden era. Standards have too, and not really in the direction people were predicting. Dijkstra did a lot of hand-wringing over the Software Crisis, which amounted to "our software is still shit." Well, it's 2012, and it's still shit unless it's touching something nuclear, but nobody seems to be willing to make the tradeoff under normal circumstances.
On the one hand, both a Ferrari and a Model-T are really fundamentally about getting you from point A to point B. The majority of the progress between the Model-T and the Ferrari have to do with unnecessary optimization (going bazillion miles per hour) and tangental/incidental value-adds (being red, having air conditioning, etc.)
On the other hand, an inablity to discern qualitative differences is exactly why every discussion about Apple on Slashdot turns into "My Rio did everything the iPod did and more! It isn't fair! Wake up sheeple!" Maybe everything we do with Google Plus we could have done with email in the 80s, but does it matter if all the end user wanted to be able to do is put their signature in lilac Comic Sans?
Where we are today (in any field, on any topic) is a mixture of good decisions, practical solutions and random selections. Every pundit (and everyone on HN is a pundit) thinks they know which things arose intentionally and which things were just noise. I hesitatingly suggest that a large portion of the intentional arose from the desire for good looking stuff. I double-plus hesitatingly suggest that an alternate world in which Plan 9's view (everything is a filesystem, every user is command-line fluent) or in which Smalltalk's view (every user is a programmer, everything is live and can be live-debugged) would be interesting and offer certain benefits. Nobody will ever really know what that alternate world would be like—besides that it would probably look like absolute shit—and I think it's unfortunate, in the bittersweet way that unrealized potentials always are.
But I take heart knowing that at least in this world, with very few exceptions, technologies can become marginalized but they can't really be killed anymore.