Complex sentence: Ironically, the person who gave the talk that day was Frederick W. Franz who, though he was some years older, outlived Knorr, and succeeded him as the president of the Watchtower Society.
Split into simpler sentences: Ironically, the person who gave the talk that day, was Frederick W. Franz, who was some years older than him. Franz outlived Knorr, however, and later succeeded him as the president of the Watchtower Society.