"No doubt, in a free world, where all human interaction is voluntary, different groups of people will live different kinds of lives with different rules and practices."

This is exactly the future I hope to help create. One where there is as diverse a system of relating as there are people to relate! That each one is tailored to it's participants! This is possible, and practical!

"However, my prediction is that even though collaborative ownership might work in small enough groups, it will break down when groups grow bigger."

That's just it, with decentralized governance models they don't have to grow bigger! We can fractal this model on any growth scale and the division of collaborative ownership will never exceed roughly 150 people before it fractals out. As roughly 150 is what has been called an optimal number of people to interrelate. It's about as many people as we can recall names, information, relationships about.