Since 1974, Beach Blanket Babylon has been a San Francisco staple for entertainment-seekers, tourists and residents alike. But the fun will end on Dec. 31, when the world’s longest-running musical revue holds its last show. “It’s just time. We wanted the show to go out on top, and we have the best cast and crew right now. They deserve the best,” Jo Schuman Silver, wife of the show’s late creator Steve Silver, said.