DOTA as well, started as a mod for Warcraft 3 and eventually spawned a franchise.
Trying to capture ALL the value (in this case the game developer/publisher) is a mistake anyway. The healthiest game franchises have broad ecosystems around them. Releasing the source code of old games just encourages that.
If there is one set of games I wish were released it is Ambrosia Software's catalog. Of all their games the only one I can find that is open source is Maelstrom (It's the Escape Velocity series, I'd love to see opened).