There could be all kinds of secondary motivations, including that ability. But most GPL'd software includes the "or later" provision, which means it's relativel

There could be all kinds of secondary motivations, including that ability. But most GPL'd software includes the "or later" provision, which means it's relatively easy for the FSF to update the license of projects, especially if they stipulate that contributions must be licensed thusly. Note that with or without assignment, the FSF could never rescind the license on previously released software, except perhaps for infringers (because of the termination clause). So assignment doesn't actually provide much value on that score except as it relates to enforcement.