Disbanded or no longer in business does not imply lack of copyright. Take for example Pumpkin studios, the developers of Warzone2100. When they decided to close down, they released the source code of Warzone2100 and made it open to the public--free to modify and distribute. The source code did not originally contain any of the video and music content of the original game. Why? Because they didn't hold copyright for that content. After some time they got permission from the copyright holders to release the music and video content with the source code.
Open source means the source code is available to everyone, to modify and redistribute. Look at
Penumbra Overture's source code. Do you see anything other than text files? Distributing anything beyond Overture's source code from Penumbra Overture would be illegal, since only the source code has been made publicly available. And as far as i know, Diablo is not open source.
Youtube, though bearing similarities, is an entirely different issue.