I think the other side of copyrights where you can irrevocably sign over your rights (which may be more of a US thing, UK seems to have some inalienable ones) is a significant issue (as mentioned, the Foster thing where Disney wasn't even paying him because they argued they'd acquired assets without liabilities - https://winteriscoming.net/2021/04/23/disney-star-wars-autho... ).

It's hard to fully balance. Another example would be Don Rosa's McDuck comics - he had to fully assign all copyright on them to Disney when making them, so the way the reprinters get him some money is by having him write forwards/commentary, those he can retain the copyright on - https://career-end.donrosa.de has more