Consider if the FSF merges with a different organization with a different charter; like the often more corporate-friendly Open Source Initiative.
I don't see how a merger between FSF and OSI would pose a problem. The Open Source Definition published by Open Source Initiative is worded nearly identically to the Debian Free Software Guidelines on which it was based, and each of the OSD's conditions maps to one of the FSF's four freedoms.
Would be nice if such a guarantee could be written into the license itself.
I agree. But given how some countries appear not to recognize a dedication of a work to the public domain as irrevocable, charters are the best we have.