John Kerry, Barack Obama, George W. Bush, Al Gore,Bob Dole, Walter Mondale, Jimmy Carter might disagree with your assessment
Apples and Oranges. Incumbents can win Iowa. And when the competition is weak, the candidate with most name recognition and money can win Iowa and run the tables to nomination.
But when there is a strong field running, such as in 2016, Iowa win does not portend the trend of things to come. Usually the candidate with most evangelical support comes out ahead, but then fizzles due to the strong field of candidates.