Rebol is 100% proprietary. To be popular it would have to offer a huge advantage over all of the other decent free languages. As far as I know this is not the case.
I think a 'law' could be made out of that principle.
Any proprietary language has to be significantly better than its rivals to gain traction, and if it does gain traction a free implementation or alternative will eventually appear.
I think I'd have to add "general-purpose" into the mix. Complex proprietary niche languages seem to be able to survive a long time without competition. Especially true when the users of the proprietary language are not themselves computer programmers and nobody has the skill to just start creating competition. (Which is why mathematics does have some open options; there's enough skill crossover to make things like Octave possible.)
Octave is decent at what it does, but nothing in the open source world comes remotely close to Mathematica. Having easy access to it (and more importantly, being able to expect my peers to have the same) is one of the things I miss most about being a student. Wolfram's recent price cut for non-student personal use is alluring, though. Some time soon I might break down and pay for it.
I wasn't claiming it was a replacement, just that it, well, exists. Other proprietary areas don't have any open source competition where the target audience has even less overlap with "programmers".
I consider "being the only thing that serves the niche" a huge advantage. Same goes for "being bundled as the only scripting language for commercial product X" (like AutoLISP, especially before VB support in AutoCAD).
I've never looked properly into Rebol and don't want to take the time to do so now. But if any HNers know about it, I'd be really interested in seeing a couple of examples of how Rebol does code=data. Specifically, is there a mapping between how Rebol represents code symbolically and how Lisp does it?
The OP did prompt me to take a look at http://en.wikipedia.org/wiki/Carl_Sassenrath, which is actually pretty interesting. I hadn't known about his work on operating systems and the Amiga.
If you watch his talk he doesn't claim he invented JSON; he goes out of his way to say he discovered it -- because it was already there. And he even says that he wasn't the first to discover it either -- as many other people figured it out around the same time.
The first thing he says is that he didn't invent JSON. He also tells who used it a year before as such. Around 8:30 he also explains why he did create a website - to create a "standard" out of it, etc..
It would be better if you watched the video before commenting this.
