The compactness of L-systems makes me think it might be a good idea to combine these more traditional types of procedural generation with AI-based generation to take advantage of the efficiency. Especially if we were trying to quickly load in something like a virtual world (rather than needing to replicate a real one). The L-systems rules are probably much more compact than the AI vectors. And there might even be some parallel versions.

What we know about our brains (whether it's a simulated brain or not) tells us that effectively our perception is a simulation of our own. Because for example as you look around, your eyes only get high resolution data in the center, and that can only represent a fraction of the relevant immediate environment. And none of the senses or motor activations are quite real time, there is always some type of lag. The brain takes the data samples and uses them to fine tune a generative prediction system that constructs the dynamic and holistic 3d scene and your position in it using the computational power of your own network of neurons.

To me, dreams provide a certain level of proof that we can do this type of simulation, and something similar is happening while we are awake, but with environmental data sampling and other cognitive processes feeding in.