Such great presidents like FDR left no real legacy in minds of most Americans, because they simply forgot about him when he died and Harry Truman became his successor, and they forgot about Truman when Eisenhower take office and so on.
Um...you lost me here. The people who lived through the Depression and WWII very, very, very much remembered FDR throughout their entire lives, whether they loved him or (in smaller numbers) hated him. The memory of FDR only has somewhat faded as those people are basically all dead now.