By the way, that means that if authors are submitting content to Wikipedia, with the intention that nobody would be able to create a derived work from their article and slap "all rights reserved" on it, those authors are putting a lot of trust in the Free Software Foundation, aren't they? Since the FSF might someday release a version of the FDL which allows third parties to create derivative works published under "all rights reserved", like CC-BY does. (Not that the FSF is ever likely to do that, obviously, but it's still unusual to have an agreement that one party can unilaterally change at any time in the future.)

The license says:

"The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns."

Removing the requirement that derived works be released under a free license would not be "similar in spirit".