Usually, the forest is used as an example. Forests are indeed highly "optimized" and full of life, but they are also mostly closed systems. What is produced by the forest is consumed by the forest. We don't want a closed system, we want a system we can tap into and feed our million-people cities. I don't know what such a thing will look like, but most likely not a forest.
The other, more specific issue is about nitrogen. Half of the nitrogen in our body comes from artificial fertilizers, extracted from atmospheric nitrogen using the Haber-Bosch process. If we stop using artificial fertilizers, how are we going to get that nitrogen?
We are almost 8 billion and counting. We are stretching the limits of what nature is capable of. And I refuse solutions that involve limiting population (i.e. genocide), or make our life miserable. Can perma-cluture (or similar ideas) achieve that? I like the idea of a more integrated system (less monoculture), how to do it with reasonable manpower is an open question though, but I also think that artificial fertilizers are a necessity at that point.