In this last year, I've used the phrase "The Forgotten Future" to represent the promises that Quake represented.
Quake was very easy to mod, very standardized and if you saw the tremendous amount of modding done with Quake, you would immediately thing the promise of the future would be easily modded games.
It never happened.
Closed sourceness. Games on rails. Hyper complexity.
[Aside: And if you think closed sourceness doesn't matter, tell that to the games I purchased in the 1990s or early 2000s, some of which I love, and next to none of them run on Vista (Unreal, Carmageddon 1/2, Moto Racer II, Judge Dredd pinball, etc.)]
But a fumbled ball is one that can be picked up.
Around 2002, everyone thought was Quake was "dead".
Is it?
Quake is dead, but dead like "In his house at R'lyeh dead Cthulhu waits dreaming."
It is going to rise again. Look at Nexuiz. It started to great acclaimation, then stuttered very badly, recovered and looks like it is thriving.
Modding needs easy gratification to beginners and Quake is very easy to map and relatively easy for QuakeC modification. And modifying the engine, as long as you stick with something not bleeding edge, is relatively easy to pick up.