Two years ago, the smart money in the orchestral world said that the St. Louis Symphony Orchestra would be lucky to exist in 2004, and that even if it survived, its downward spiral of debt and disorganization would cost it its place in the top ranks of American orchestras. Since that dismal time, the SLSO has scrambled back from the fiscal precipice, shored up its organization, and, this week, hired one of the most celebrated young conductors of his generation, David Robertson, as music director. With Robertson on board, the orchestra is convinced that it will shortly complete one of the great comebacks in industry history.