Paradise Lost takes place almost exclusively in a time and place when death, sin, and lying didn't exist. In other words, it deals with a time when humanity was still innocent. While Milton is very good at painting a picture of innocence, he realizes and reflects on the difficulty of attempting to describe something as alien as a deathless, sinless existence.
Just because Adam and Eve could have kept Paradise forever does not mean that God wanted to keep them totally innocent; Raphael makes it clear that there would have come a time when they achieved some degree of experience.
Milton's attempt to describe innocence ultimately fails, if only because he must present it through the fallen medium of human language.