I think Mark and mlkluther have touched the heart of the thing: the question of “what if” is not answerable from the Genesis 3 narrative alone, but only in the context of the broadening scope of Scriptural revelation (Genesis -> Torah -> Tanakh -> whole Bible), and in particular christology.
All those not only show the role of Christ in both salvation and creation, but also, by the way the story has panned out, they illuminate what the purpose of the garden, and of Adam, was in the first place.
In Narnia C. S. Lewis always has Aslan say, “Nobody is told what would have happened,” and that, I think, is his nod to the deep providence of God - the Incarnation was always in the eternal plan of God, but it remained within his counsel alone until it actually happened. The game was real.
Still, for it to be real, “what if?” questions must be answerable in principle, if speculatively. It’s not a soccergame if there’s only a goal-mouth at one end because everybody knows Team B is too rubbish to score.
The temptation in the garden had the character of a crucial test of covenant faith and loyalty. It was not simply “a” man committing “a” sin. And in the light of Trinitarian theology, that faith and loyalty were to the Son, who is God as he relates to his creation. Adam and Eve walked with the one in whose image they were created.
Had Adam and Eve acted upon faith regarding that command, they would have learned wisdom, and grown in love and righteousness, from the source of wisdom, love and righteousness, and the “Eden economy” would have spread to fill all things with glory through humanity’s fullness, through God’s Spirit.
That is seen through the continuation of that “new creation project” firstly in the call of the Patriarchs leading to Israel (and their failure), and finally in Christ pulling the whole thing together in his Incarnation and Passion. The whole salvific aspect of Jesus’s ministry is therefore preparatory to getting the larger, original project - new creation through humanity - back on track.