MILAN -- Sergio Marchionne bowed out on Saturday as one of the auto industry's most demanding and tenacious chief executives, his health in crisis after a career in which he rescued Fiat and Chrysler, two of its most storied brands. Fourteen years after he first took the wheel of Fiat, the gruff 66-year-old was replaced as boss of the Fiat Chrysler Automobiles (FCA) group he built. He had suffered serious complications from shoulder surgery and his health was worsening, the company said.