Drusilla makes an essential correction to my point below about the completeness of Adam and Eve's knowledge of good and evil before the Fall:
Be careful not to take a short term view of creation – a view that insists there was nothing more for A&E to learn, that they had no need to grow until after they ate of the fruit. Growth seems to be built into creation (cf. Romans 8 and elsewhere)...
Obedience is also an opportunity to choose that which has been bestowed, an opportunity to participate in that which God is doing; it makes love something more than nice, warm feelings. There is reason to believe that A&E, though in a blessed state of sinlessness, had not yet grown up into the fullness of what it means to be made in God's image.
This makes a great deal of sense to me: If Adam and Eve had never sinned, they would have continued in innocence but they wouldn't have remained unchanged in wisdom.
I don't think that directly contradicts my suggestion of the completeness of their knowledge, since it was (and, without the fall, would have continued to be) all they needed to know at the time. The "shortcut" to wisdom, as Drusilla put it, that Eve tried to take with the forbidden fruit knocks them out of this state of fullness. They suddenly have more knowledge (e.g., their nakedness) than they have wisdom (fig leaves?), and they must be expelled from Eden. We might suppose that, if they had eaten of the tree of life at that moment, theirs would be lives forever be out of balance.
This idea seems to help with a few difficulties I've had with the myth. One was the impression of the pointlessness of the whole Garden environment. From eternity, God knows Adam and Eve will disobey Him; the trees and the naming of the animals and all that seems like an awfully elaborate set-up for something that will be discarded even before their first child is born.
But if they could have continued to grow in Eden, then it becomes a fully realized place, a Paradise that could have sustained mankind forever, not just a Potempkin rest-stop between the void and our current vale of tears.
It also resolves the accusative dimension of the cry, "O happy fault!" If our state of union with the Trinity will be greater on the Last Day than was Adam's in Eden, then God could be accused of coming up with a highest and last good for us that would only be possible if Adam sinned. That's like saying, "If you play fair and win, you'll get a hundred dollars. If you cheat and are disqualified, I'll reinstate you and you'll get a thousand dollars."
This way proposes a Plan A, which would have gotten the human race to the same place we're headed for with such and so great a Redeemer.
Clearly, Plan A would give the Incarnation quite a different purpose. As St. Thomas writes elsewhere, "the fellowship of friends conduces to the well-being of Happiness," and Christ might become man as friend -- or better, as Bridegroom come to wed His ever-spotless Bride.
I don't know, obviously. But it does make sense to me.