@John
Wouldn't substituting blackberry with thornless blackberry work? You could either go napalm and switch or gradually introduce the target species.
I think people can get hung up on the 'do-nothing' and the 'let nature decide' ideas. In my plot, if nature was deciding it would grow a field of grass and goldenrod > toward
black locust > toward oak, chestnut, hazelnut. I keep goldenrod and grass, but they get seriously limited in their ability to express themselves. Technically, I'm part of the nature there, and I'm deciding to have things I want without creating a huge disturbance. I have zero cognitive dissonance about this.
Anyway, finding natural equivalents is useful. Goldenrod grows, and so that means sunchokes will also grow well, which they do. Most weeds have their cultivated counterparts which are good choices for substitution.
The other is finding where nature is heading and push it in that direction. That is 'doing nothing against nature' which is the original meaning of 'do nothing'.
In an urban backyard there is so much that is a result of human interaction that finding out what nature would do is more difficult and might change from one meter to another, as conditions can be vastly different. You're working within a situation in which nature is guided substantially by the human element which can be for the better or the worse, depending on the human. I recently let a shady urban garden go, mint and cinquefoil took over. It really depends on your own needs and what the site has to offer and your range of possibilities for substitution.
William