That would have been my guess too, but it doesn't preclude the experiment. I personally think there's more to it than pure enforcement. Coherent standards for what counts as an invention, and a well defined expiry term, are two things that attracted adherence to the patent system. Without them, patents would be too much of a mess to enforce. Those things have evolved over time within the existing patent system, and depend on not only statute but also case law. I don't see how anything like that can build up independent of having a robust government operating over the long term.

On the other hand we already have "copyright without enforcement" for recorded music, via file sharing.

Disclosure: I have 20 patents.