What do you think he meant when he said, "If I've lost Walter (Cronkite), then it's over. I've lost Mr. Average Citizen." Explain.

He meant that, he placed so much faith in Walter Cronkite and valued him. That, he cannot afford to lose the relationship and frienship between him and the Walter.

Losing it on the long run, made him to feel that he has lost everything hence his comment about having lost.