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).
There is little chance of FSF changing the GPL as long as RMS continues to run it, given his monomaniacal, pathological obsession with software freedom at the expense of literally everything else, including basic human decency.
But he's not going to live forever (thank god) and his successors might try to use GPL4 as a political wedge within the FOSS community. Put in restrictions on project governance and codes of conduct, make a violation unrelated to the software itself terminate the license, something like that. "Ethical source" didn't get anywhere but something backed with the heft of FSF might. (Given how RMS has stacked FSF with loyalists, it's probably more likely that they'd do something to punish "ethical source" than enshrine it. Both are equally corrosive in my view.)
Or it might make FSF even less relevant. We'll see.
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