And the biggest problem with your logic is that although ‘controlled airspace DOES NOT stop collisions’ (and nor do traffic lights), controlled airspace (and traffic lights) are more effective than nothing in preventing collisions. It’s a simple concept: In which airspace would you prefer your loved ones to be carried in the back of a 737: G or some other classification?
I could be wrong. It may be that we’d all be safer if all airspace were G.