Excellent point, Godfrey .
Stephen Greenblatt also argues against a seventh book, on the grounds that the end of Book VI sets up a test for the reader to see how well they've grasped the main points of the philosophy.
George Santayana suggested that the poem was unfinished because he anticipates that Lucretius would have ended the poem with Mars to complete the symmetry of beginning with Venus.
Here is a thread on the plague at the end of the book, with a probably meaningless anagram I discovered: Mortifer aestus: "A deadly fever" -> Fetus ore Martis; "Offspring from the mouth of Mars."