Heaven may not be a possible world when you take it in isolation by itself. It may be that the only way in which God could actualize a heaven of free creatures all worshiping Him and not falling into sin would be by having, so to speak, this run-up to it, this advance life during which there is a veil of decision-making in which some people choose for God and some people against God. Otherwise you don't know that heaven is an actualizable world. You have no way of knowing that possibility.