Despite a spell of initial optimism in the 1960s and 1970s w

Despite a spell of initial optimism in the 1960s and 1970s when it appeared that transistor circuits and microprocessors might be able to copy the action of the human brain by the year 2010,researchers lately have begun to extend that forecast by decades if not centuries.这里的后面部分翻译是:这个预测往后推迟了数十年甚至数几百年.请问这里的by和if not怎么解释?