If God intervened in the garden and wiped away sin for good, then there wouldn't be a story of humanity's freely worshipping him. There wouldn't be much of a story at all. Because God allowed choice to shape our experience, we can freely love him. Lewis says in our next day's excerpt:
... freewill, though it makes evil possible, is also the only thing that makes possible any love or goodness or joy worth having. ... The happiness which God designs for His creatures is the happiness of being freely, voluntarily united to Him and to each other in an ecstasy of love...So, "The Holy Eraser" is not a swooping down God who makes people subservient to him against their will. Rather, as we see in the introspection of Lent and the conclusion in the cross, Jesus provides the miracle of new life. Perhaps this is why the New Testament writers talk about Jesus as the "new Adam." He repairs what is broken, mends the wounds of sin so death has no sting.