Should game developers release the source code and other assets of a game when they go out of business?
It's always been the case that, aside from the employees of a defunct game developer, it's the players who lose out when a game developer turns out the lights in the server room.
Not only do they lose the characters they've created when the game servers go down, they also lose the relationships that they've built up in the game, and the sense of community that MMO games encourage.
Over the years there have been a bunch of MMO games that have closed down. Earth and Beyond, Auto Assault, and Asheron's Call 2, to name but a few. None of the developers or publishers behind these games, to my knowledge have ever released their MMO game as open source - despite, in some cases, calls from the player community to do so.