Somewhere, perhaps only in my own fevered mind was mention that while the vaccine was supposedly to provide longevity to all in actuality it was to get the ball rolling on phase 2 of a draconic servitor race. Lofwyr destroyed it in part because he felt the drakes were a terrible idea (and he should know) and didn't want to see the same mistake repeated.
Alternatively L could have destroyed it because giving everyone longevity is basically an ecological disaster waiting to happen. Global population, human extinction, to paraphrase Kosh, we're not ready for immortality.