IANAL but the copyright assignment is probably necessary for the FSF to have the rights to change the license at will (within the limitations allowed by the copyright assignment). If there are many copyright holders, like for say the linux kernel, a change of license requires the approval of at least all major copyright holders, IIUC.
This is incorrect, anyone can upgrade the license for GCC (and the rest of the GNU project), since GCC is licensed under the `GPLv3 or any later version'. Linux on the other hand is explicitly licensed only under GPLv2; i.e. it lacks the `or (at your option) any later version)' clause.
