You don't have to do it in real time, per se. I imagine a world in which the renderer and the world generation are decoupled. For example, you could descriptively articulate what you wanted to achieve and have it generate a world, quietly do some structure from motion (or just generate the models and textures), and those those as assets in a game engine for the actual moment to moment rendering.

You'd have some "please wait in this lobby space while we generate the universe" moments, but those are easy to hide with clever design.