Modern works can fall into public domain if there's no one who can make a legitimate claim to them - due to neglect or lack of heirs, or sometimes even just a screwup. For example, for bizarre arcane legal reasons some episodes of the Dick Van Dyke Show are public domain but most aren't. Night of the Living Dead is public domain because they didn't put a copyright notice in the credits (the law at the time, but not any more). It's a Wonderful Life is sorta half-public-domain (but not entirely because the story it's based on is still under copyright.)
On the other hand, Disney (and most conglomerates) not only carefully maintains their copyrights but as said above, have won legal battles to extend copyright on their valuable properties far past their original expiration dates.
btw, I just filed the copyright papers for Return of Pink Five Volume Three yesterday - so it's my death +70 years before any of youse can get your meathooks on it. Ha HA.
