Copyright assignments made by authors can be cancelled [1]. For papers and books from before 1994 or so (25 years from present), you can write a letter to the publisher cancelling your grant or assignment of copyright to them. The rules are complex so I might be missing some details. But there are probably a lot of researchers who could take advantage of this to put on some pressure and reclaim their work.

[1] https://www.copyright.gov/docs/203.html