Maybe in the distant future, a pack of game developers could create an open source engine. It would have to be something like RenderWare, so it can be used for any type of game (fps, rgp, race, mmo, etc.).
At this moment, that's not the case at all. Unreal Tournament 3 was a forefront to sell the Unreal 3 engine, Crysis 2 to sell CryEngine 3, Half-Life 2 to sell the Source engine and the upcoming game Rage to sell the id Tech 5 engine. Epic Games, Crytek, Valve and ZeniMax make a lot of money licensing engines.
Although ZeniMax has of course released id Tech 3 before five years after Quake III Arena. So maybe we can expect the same with id Tech 4 in a few years.
Most open source engines are modifications of the id Tech 3 engine, or can't reach the technical level of previously mentioned engines. Like
Cube 2 which is pretty good, but lacks decent physics for example.