[pilaf]

id Software has a history of releasing the source code to their engines ~4 years after their initial commercial release:

Doom released in 1993, its engine was open-sourced in 1997 (4 years).

Quake released in 1996, its engine was OSS'd in 1999 (3 years).

Quake II released in 1997, OSS'd in 2001 (4 years).

Quake III released in 1999, OSS'd in 2005 (6 years).

Sure, in most those cases they were at least 2 generations of their own tech ahead when releasing source code, but considering how they were at the cutting edge of game development at the time, arguably those engines still held considerable value at the time and far from worthless.