The code and logic of games can be open source (look at Dwarf Fortress or ports of Doom) while the assets are still closed source. The bigger fear I always see

The code and logic of games can be open source (look at Dwarf Fortress or ports of Doom) while the assets are still closed source. The bigger fear I always see is that someone will steal the code and run it without paying. In reality, this almost never happens. People don't care enough to run their own version of the game, or any other software.