The FSF is at it again - claiming that usage of the GPL is on the rise, when its' share of the market is declining, both in F/LOSS, and in the larger software ecosystem.
So, time to let everyone in on a little secret - any gpl'd project can be taken closed-source by anyone willing to go through the exercise.
Copyright law only protects a limited portion of all creative works. What I mean by this is that neither portions of copyrighted works that lack creativity, nor those parts that are "scenes a faire" ("there's only one way to do it") are protected. APIs, for example, are one such "scenes a faire".
Remember the "linux headers" FUD the FSF put out? Even Linus agreed that the headers, simple macro definitions, enums ... they simply are not protected. The same rules applied to Google using Apache Harmony - java class names and method signatures are not protected. They either lack the necessary creativity, or there is only "one way" to do it.
1. replace all artwork, comments (comments are expressive, and as such, protected by copyright);
2. rewrite all function bodies that are not "scenes a faire"
3. PROFIT! (maybe).
You can release the result under any license you want - and you don't have to distribute your source. Better yet, you also maintain binary compatibility with the original.
Business A develops GPL software and sells support. Business B doesn't have the overhead of developing that software, so they spend the money and resources saved on things like marketing the crap out of how they are better at it, and developing a few plugins that require server-side services that only they provide.
So Business A says "the heck with this", does what I propose, forks their own software, and releases a new closed-source version that breaks only Business B's code.
Why wouldn't they?
More importantly, why wouldn't B do this first, as a preemptive strike? Once you have a "good-enough" code base, you don't really need community support for further development. In fact, releasing code "to the community" is now where software goes to die. It's the digital "elephants' graveyard."
There's really nothing legally preventing anyone from doing this and being able to sell the resulting code over and over again. Both businesses and consumers are used to that sort of arrangement.
So, can we expect to see a linux "clone" by the end of the decade? I doubt it - there's no need. BSD already runs linux programs. But I do expect to see closed versions of many open-source programs pop up once a few test cases make the rounds.
It's already being done
Sony is making a busybox clone, and there's nothing that can be done about it. So, people have a choice - do it themselves before companies like Sony do it and reap all the profits or stick their heads in the sand. In the age of "good enough computing", if it's "good enough" to clone, it's "good enough" to take private.