There is zero historical evidence to support that opinion.
Truman was strongly contested in 1948 by Strom Thurman. Truman as the incumbent did not win the nomination until the Convention. There were a lot of people in the party that thought Truman could not win. In fact the southern Democrats were so angry that they walked out and formed a third party behind Strom Thurman.
Tomas E. Dewey on the Republican ticket was totally unopposed for the nomination. Dewey had been the candidate in 1944 and came fairly close to winning. The Republicans figured that if Dewey gave Roosevelt a good race and they were united and the Democrats divided, Dewey could win the presidency easily.
And as Dewey always said "Our strategy of having me named the nominee early really worked..... to re-elect Truman!"
In 1952 the Republican Convention was very contested with conservatives wanting Mr Republican, Robert A. Taft to be the nominee and the moderate Republicans wanted Ike to win. It was a knock down drag out fight which Ike finally won at the convention.
The Democrats in 1952 united behind Truman's choice of a successor, Adlai Stevenson. The Democrats were certain that a campaign united behind Stevenson would defeat a divided Republican party.. But Ike won in a landslide.
In 1960 The Republican Convention was not very contested. Everyone knew that Ike's VP was going to get the nomination. However Kennedy faced stiff resistance to his nomination. Democratic Majority leader LBJ was nasty oppostion. LBJ's campaign accused Kennedy of having Addison's disease and said his medicine effected his brain. It was a knock down drag out fight. When Kennedy won he sent an aid to ask LBJ to be his VP. Kennedy did not expect LBJ, who was majority leader, to accept. But LBJ did and Robert Kennedy wanted JFK to take back the offer. There was lots of bad blood both before and after the Democratic convention in 1960. And Kennedy was not a certain nominee until the Convention voted.
It was thought that the United Republicans would defeat Kennedy in a divided Democratic party. But Kennedy won a close one.
There was a huge contest in 1992 as no one took the lead for the Democrats early. Bush had no opposition and many Democrats felt that all the animosity in the Democratic party plus Ross Perot in the mix just might reelect Bush. But Clinton won.
McCain contested Bush in 2000 while Gore was unopposed. but Bush won.
You made a statement that has no basis in history.
There is a lot of evidence that an uncontested primary in a party whose candidates are not all that well known, gives the better known other parties candidates a much better chance at victory.
The only way the public at large can learn about this years Republican Nominee is if there is a real contest for the nomination. If it goes undecided until the convention,then the convention audience will be huge. If the nominee is already known, NOBODY BUT THE JUNKIES WATCH the Convention.
You have it exactly wrong.
It would be the best thing in the world for us... no matter who won. Unless, of course... it was McCain.. who, I will NEVER, EVER vote for.