When General Motors recruited Johan de Nysschen to lead Cadillac in 2014, the prominent auto executive hoped for a cavalry of new sport-utility vehicles to shore up the luxury brand’s weakest position. As it awaited reinforcements, Cadillac soldiered on with a legion of widely praised sports sedans. But Cadillac has parted ways with the man once seen as the brand’s future and the automaker is still seriously lacking in SUVs.