Well first off if it is all a metaphor it would be news to the people who wrote it. When Paul was talking about the Garden of Eden it is pretty clear he saw it as a true and literally described event. You can't have a physical resurrection atone for a metaphorical fall from grace. The cost is nowhere near the gain. If the Fall was a metaphorical event you could just have a symbolic sacrifice, i.e. no need to physically torture someone to death.

Secondly it makes your god a coy liar. As a whole the BIble is pretty clear when there are metaphors and when it is literal. Go slug through the later OT prophets when god is demanding people do weird stuff and telling them "this symbolizes my relationship with Israel" or "this symbolizes what the future holds" or even in the Pentauch you see it. The authors were not content with just dropping some symbolism and running off, they had to explain what it meant.

Third it just isn't appealing. Satan to me was a real threat. I can still remember how haunting it was when I first heard "call us legion for we are many". And what the terrifying implications of that meant. A city against God with forces working together. If all of this is just people creating anthropomorphic figures then well... what's the point?