That's an interesting take, but it opens up these questions-
If woman was already around, then what is the point of the story of man being lonely, and God making him a woman from his rib?
Adam was supposed to be the one who named the animals. Are we really supposed to believe that no one had named them until 6,000 years ago?
God 'cursed' the snake and made it crawl on its belly. What was it doing before 6,000 years ago? Flying?
Adam and Eve didn't realize they were naked. In a world full of clothed people, they didn't realize they were naked?
If we are paying for the sins of Adam because we are his descendants- does this mean most of the planet doesn't have to worry because it's more likely we're the descendants of other people from that time? I guess this would depend on whether people thought Noah's flood really happened, or was a metaphor. If it did (it actually didn't and that can be proven) then we'd all be descendants of Adam, yes. (Unless they had offspring with people outside the genealogical line- like Cain may have).
Did people between 6 and 4,000 years ago really live such long lifespans? 895 years, 777 years, 969 years, etc. Doubtful. Before the 'original sin', was everyone living such long lifespans?
Were people without pain in childbirth until 6,000 years ago?
If God was testing this one couple and they failed his test (which he would have known they would, seeing as this is God!) why put hardships on the rest of the humans around at the time who didn't fail him? He wants people to be good, yet even if they were, their efforts are in vain because God tested the ones he knew would fail. Kinda goes against the story of Soddon and Gomorrah, where God says if even one person can be found worthy, he won't take drastic action.
If the garden of Eden was a real place, and had to be guarded afterwards by cherubim and a flaming sword, sometime after it must have just disappeared. If it was going to disappear anyway, why not just make it disappear when Adam and Eve sinned?
People had already made up gods before 6,000 years ago. Why did the real God wait so long before turning up and saying 'Hey guys, I'm the real one. I know I've let you worship other gods for thousands of years because you knew nothing of me, but now I'm a bit angry about it, so stop. Oh yeah, don't murder either. What, you don't like murder already and you wouldn't have got this far if you did? But I'm the moral law giver! Ah screw it, I'm killing you all for talking back to me!'
The Genesis account isn't a metaphor- a metaphor for what? That story and reality don't look anything alike. The simplest and most obvious explanation is that the Bible is a made up story by tribes that couldn't understand what they saw around them.