Speaking from my experience with New Brodgar (I can't speak to Yolan's incarnation), the issue was that things stagnated, people left, and its literally impossible to give "real-time" protection to people in the city. The only effective defense is a strong wall.
tl;dr
People will unavoidably die and quit without walls, often. Once we installed walls, it got super boring, and people quit.
I am not saying its impossible, but without a fundamental game mechanics change, it doesn't seem likely to me that there will ever be a long-lasting open city.