Indeed. Kevin McClory famously held out to his share of the rights for Thunderball until he died, after which his heirs sold their stake to EON and the Broccolis.
As for Fox et al, Disney’s current strategy seems to be siege warfare: i.e., making non-Disney Marvel film franchises unviable by actively destroying their comic-book counterparts.
Last I heard, the Fantastic Four were still in the realm of “cannot be published”. After all, that might lead to a company other than Disney making money at the box office.