...Second, since the FSF owns the copyright on most of the GNU code, it
could freely change the license tomorrow to require any operating system
distributed with GNU code to be called "GNU/something".

That’s an interesting possibility to consider. Of course, the probability of that happening is almost as small as Windows ever being released under GPL 3, but if we suppose it happened, will the free software community fork all GNU projects?

That did happen with X and the fork of Xorg, but this is GNU (and, in connection, FSF) we are talking about here, not X consortium. It would be almost like comparing possibility of a coup in the U.S. against the possibility of a coup in Serbia.