The beatific vision is not the only way to prevent evil. The human will does not sin or do evil out of necessity. It can freely do good or do evil which is why it is called a free will. Adam and Eve did not sin in the garden of Eden out of necessity, they freely chose to disobey God’s commandment. Our first parents could have not sinned, they chose otherwise. On earth, we have the possibility, not the necessity, of sinning and abusing our free will. In heaven, our free will will be perfected because we won’t have the possibility of sinning any more. Sinning is a defect of the use of our free will. Also, our Blessed Lady did not have the beatific vision while living on earth but she remained sinless her whole life long. Sinning is not natural for us, it goes against human nature. Virtue is what is in accord with our human nature.