My theory on Unix's one chance to have a standard GUI
In an earlier entry I discussed my view on why Unix vendors never got together and created a standard GUI, the way they created POSIX. It's my view that the Unix world had basically one chance to create a standard GUI but fumbled it through (vendor) greed, although sadly understandable vendor greed.
Let's start with a question: why does Unix have a standard graphical environment? Because it does; for at least two decades now, X Windows has been the ubiquitous Unix way to do graphics (although the appearance and behavior put on top of it has been highly varied). And it's not because X had no competitors; rather the contrary, it had a lot. Most of the leading Unix workstation vendors had their competing graphical systems (which the vendors typically liked more than X, to boot).
What made X Windows successful anyways was that it was free and widely available for various Unix workstations. That it was free and available to users gave it ubiquity, especially in the early days before vendors liked it very much; that it was free to vendors made it ultimately cheaper for vendors to build on it (and contribute to its development) than to build everything themselves.
The difference between X Windows (which became a standard) and Unix GUIs (which didn't) is that no one ever set up an equivalent of the X Consortium that gave away a GUI for free. All of the various Unix GUIs were restricted in various ways, with no one that was as widely and freely available as X itself. Motif licensing was especially egregious and counterproductive, with the OSF doing every bit it could to make as much money as possible; my memories are that even the Motif runtime libraries were often an extra-cost Unix vendor add-on. This both reduced the development of local software that used Motif (places that didn't have a Motif runtime license used other toolkits) and significantly limited the usefulness of free software that used Motif.
(LessTif was a fairly big deal back in the days when Motif still mattered much.)
So my view is that the one way for there to have been a standard Unix GUI was for Motif to have been donated to the X Consortium, royalty-free. It might not have been a great GUI (although the Unix competition wasn't either), but it would have been ubiquitous and the ongoing development cost advantage alone would have made it hard for any single vendor to compete (GUIs are expensive to build).
(Somewhat to my surprise I discovered that OSF Motif actually is an official standard in the course of researching this. That the API is standardized doesn't help in practice, because the expensive bit is implementation.)