Mankind was always destined to enter the new earth knowing about good and evil. I don't think the first generation falling was part of the plan. If 930 years was how long Adam lived in a world where grief was common was it a worse fate that those two never sinning yet seeing all their children die for 6,000 years? Without that sin mankind would have eaten from the tree a few days before the tree of life was there to be eaten from then the next event would have been the new earth in Re:21. Plan 'B' is the contingency one, would you rather there was no Plan 'B'?