The average street attacker is normally totally naive and utterly unprepared
I think this is a very dangerous assumption to make. A lot of criminals, petty or otherwise, are well-practiced, lie in wait, and/or work in teams (pre-meditated, even choreographed attacks). At the minimum, the street attacker will likely be more mentally prepared to do harm than the victim -- with or without martial arts training -- is prepared to address it. Oh, and the idea of a chain-punching street attacker is kind of odd (rather: "many of them never see the knife").