"If I've lost Cronkite, I've lost the nation" is something Lyndon Johnson supposedly said, except he was at a dinner party that night and never said it. Public opinion had begun to turn against the war long, long before the media did. The media was towing the government line for quite a while. Meanwhile, it had gotten to the point where everyone knew someone in the war, or knew someone that did, and those people were often dead or returned home different than they had left it. The media was late to the party when it came to opposition to the war.
Cronkite saying the war would end in a stalemate was nothing radical nor did it lead the nation to oppose the war. The nation was already turning against the war. The myth is only to simplify a very complicated time, and is often part of the stab-in-the-back mythology. "We could have won the war if not for the media and the hippies".
Beyond that, Walter Cronkite dying would have a definite impact.