No one can prove a negative. Or maybe in this case disprove a positive.
Just because a highly exposed candidate did not win, does not negate the power of that exposure. ie. no one knows how much worse they would have done WITHOUT that exposure.
And yes, Trump is a good example. He did not lose (by the rules in place). He won. And he did it partly from all the free media coverage. He was the most covered candidate by a long shot. Free airtime his campaign could only dream about. Do you think he'd have done BETTER if he had not had all that free airtime to blast out his lies? To the Rust Belt for instance, how he was going to bring their jobs back, and how Hillary and her husband killed their jobs with the NAFTA? If less voters heard his fake promises, they'd have voted in even greater numbers for him? Really?
Anyways, this argument is dumb. Its like arguing that advertising does not work, because you are still second in sales in your particular industry. That major companies must be stupid, and are wrong about spending on advertising, and should just save their money. Especially if there is a more successful company they compete with in the same industry. An executive saying..."look X company beat our ass good again last quarter too...it must mean that we should stop all advertising and other types of community exposure to our product!"