When strong commercial encryption becomes illegal, two things will happen: 1. Criminals and terrorists will construct their own strong encryption (anyone can buy books explaining how to do this from Amazon, and any competent programmer can do it) and 2. China, Russia and organized crime will penetrate the weakened commercial encryption.
"When strong commercial encryption becomes illegal"... criminals and terrorists will continue doing illegal things. They will not say "oh shit, encryption is illegal I better stop and be good now".