Even if something so absurd would happen, the version of the code from just before the change to a new license would still be available under the GPLv3. So everyone who wouldn't agree with this change (which would presumably be everyone), could simply fork the code from that point and continue under the GPLv3.

The more realistic risk of a CLA on GPL'ed code is that the person in control would switch it to a more permissive license without the copyleft restrictions of the GPL. That would actually have a negative impact that others can't mitigate by forking (although with a fork you could of course again make any new code added to the project the original license again).