SUCCESS of the campaign to rebuild American downtowns has confounded those experts who repeatedly proclaimed the death of city centers after World War II.  As recently as the early 1970s many old downtowns still looked like newsreels of the Great Depression; businessmen, public officials, and academics all agreed that downtown was finished, and the central city close to collapse. But not long afterward, the noise of jackhammers and pile drivers drowned out the chorus of gloom.