Just some musings:
I think there's more to measuring a human (and society) than risk reduction. For example, the degree to which a human is mentally prepared to confront risk, and the skill that a human has to navigate risks.
I train parkour. That training has certainly influenced the way that I measure mankind (as you can see above), but I've also found in it a fantastic tool for identifying and pushing my physical and mental boundaries, which I'd definitely call advancement.
Playgrounds, as it happens, provide some of the highest-density areas for finding body-and-mind challenges. Overly safe playgrounds, however, severely limit the number and degree of challenges, and as such make it a lot harder to push oneself in a progressive fashion. (In my case: sure, it's possible to train in a gymnasium or around the city, but the former tends to be too safe -- there aren't many gymnasiums out there designed for parkour -- and the latter tends to be too risky for those who haven't worked themselves up to that level.)
Zero risk is an interesting concept, but I don't think we'll ever achieve that (just an axiom in my mental model of the world), so one must be prepared to confront it. Eliminating that risk from playgrounds, in my opinion, weakens one of the best tools we have to build up those skills.
I'm not an advocate of exposing children to murderers, thieves, and rapists to "toughen them up"; that's just silly. I'm more interested in expanding human ability (specifically, my own) to deal with the universe. Risk reduction is one way to do it; having the physical and mental training to overcome that risk is another. I don't think they're mutually exclusive.
