The entity holding the copyright can change to a new restrictive license, and continue development there, effectively killing the old GPL version and so locking down the project.
The original contributors would not agree, but they gave up their rights.
However, I think for the entity to do that in practice, the entity would need to also own the trademark and the Github repo (or wherever development takes place). So there's no real risk to assigning copyright to the FSF if the FSF doesn't also own the trademark and the Github repo.
I don't disagree with the general claim, but about your scenario specifically - the "entity holding the copyright" is not, generally, the entity doing the development. If it is, then the question is not copyright assignment but just whether or not the main developing entity sticks to a FOSS development or not.
The original contributors would not agree, but they gave up their rights.