What can be said?
John Carmack is a purest. He designs engines the way they "should be done" rather than how mucking about with tricks and hacks and Graphic Chip specific gotchas. He complained, even after Doom 3 was released, that the hardware vendors didn't follow his advice about future workloads. In his mind, ATI and Nvidia got it all wrong. I kinda agree, if you look at all the modern engines they all do deferred shading which is managed by SOFTWARE culling. Nvidia and ATI should be handling that stuff, not Epic and Crytech.
Doom 3 was double tragic for Carmack because it squeezes the most out of a Geforce 3 that you can but it launched too far after the Geforce 3 was relevant. If he retooled the engine for better DX9 usage, all those folks who paid $1000 for an overclocked Geforce 3 would be throwing temper tantrums over the broken promise. So instead he kept his word and the engine renders just about it's full feature-set on a Geforce 2
leaving DX8 and 9 level shaders for post-processing stuff only.
Then, adding insult to injury, OpenGL became a big clusterfuck because ATI and Nvidia could never agree on how it should go and the "oversight committee" were a bunch of spineless pussies who couldn't arbitrate worth shit. It's lucky that Doom 3 even had DX8 level features to work with by the time it was released. OpenGL 2.0 was barely above DX7 feature level and half of it's features were broken in both ATI and Nvidia drivers because of the ongoing tug-o-war for OpenGL's future (or intentional $abotage by Micro$oft???)
Hopefully, the GPL release will be a chance to rewrite history and use the engine to it's fullest potential.