"Also - study the code of the likes of Carmack. Consider that he produced the likes of the quake engines in only a couple of years. Reflect long and hard on the raw simplicity of a lot of that code."

Also says something about the accumulation of complexity. At that time Carmack (and his team) were able to create a state of the art engine in a few years. Now consider the task today, if you were to create a state of the art engine today it'd take tremendously more work.