For 2000+ years every Christian and Jew takes the Bible at face value. Suddenly in the last generation or so, it's all an allegory of a symbolic metaphor. The trouble is, once you assign it to symbolism, then it can have whatever meaning you want.
I think you're wrong on the facts here. It is not true that historically every Christian and Jew believed in the literal truth of the Bible. Throughout history there have been many, including many church fathers, who viewed biblical tales allegorically rather than historically.
My own take has long been as follows: the gods, fearing mankind, tried to keep certain knowledge from us through lies. The serpent, pitying us, revealed the lie and gave the knowledge to mankind... and for it he was punished, just as Prometheus was in Greek parallels. Moreover it seems clear to me that the first tree offered knowledge not of "good and evil" in general but of sexuality in particular, which explains both their response in covering themselves and the form of Eve's punishment. With our new knowledge of sexuality and the ability to reproduce, the gods feared that, if we also gained immortality, we would become more powerful than they, so they banished us from Eden to prevent our reaching our full potential. It's not a story of our fall from grace by doing evil, but of jealous gods keeping us under their thumb. I do not doubt that the serpent was originally the hero of the story.