> Things like the famous fast inverse square root are short, but I would hesitate to describe it as simple.

Not the best example. That snippet was in use at SGI for years and actually written by Gary Tarolli. Quake's optimization was mostly done by Michael Abrash.

The original id engines were also famously inflexible. They fit the mold of "developing an engine, not a game" to a T. What you saw them do was all they could do. Look at how much Half-Life needed to add to be viable. idtech3 also only broke out of its niche because Ritual and Infinity Ward heavily modified it and passed it around. There's a good reason the engine-based ecosystem is so prominent now.