I wish this were further up in the comments. Most people seem to be assuming that you get all the marbles back.

Imagine the chaos if someone were able to say ‘whoops, all those books you bought are no longer sellable!’.

Imagine if Alan Cox took back all the bits of Linux he wrote and decided they were no longer to be licensed under the GPL!

…although maybe it’s only a matter of time before that second thing happens somewhere? “The company may not prepare a new movie based on the novel; it may only continue to use the existing movie that it created when the exclusive license was still current” seems problematic for open source software (or commercial software! What if the original authors of the FAT file system decided to try to start getting royalties from new derivative works?…)

Replying to myself…

From https://rightsback.org/faq/#What_kinds_of_agreements_cannot_... , works for hire are not covered, which includes most 1990s software (though I do still wonder about open source licenses):

Under the U.S. Copyright Act, copyrighted works that qualify as “works made for hire” are subject to special rules that govern who becomes the first owner of copyright in a work. For regular works, the person who creates the work becomes the first owner of copyright. However, for “works made for hire” either the employer or person who commissioned the work becomes the first owner of copyright. Neither of these transfers of rights from the author to the employer or commissioning party, which occur by operation of the Copyright Act, nor any subsequent agreements entered into by the employer or commissioning party in relation to the work, may be challenged by the author or their family members.

There’s a whole lot more nuance there, but notably “A contribution to a collective work” is allowed to be a work for hire.