In many cases, it is clear le-olam does not mean "for ever". There are many scriptures that make a lot more sense if le-olam, and the Gk. equivalent were understood as meaning "unto the age" or "for an unspecified amount of time", for example when used in Jonah 2:6 to describe the age/epoch/time of Jonah's stay in the belly of the seabeast (both Olam in Hebrew in the Masoretic Jonah, and Aion in Greek in the Septuagint Jonah are used in this sense.)
Scripture also says many things would last "le-olam", which have not lasted forever. For example it says the sons of Aaron would offer sacrifices le-olam, which historically can not mean "for ever", since they are not currently doing so.