Can you imagine how people would react if they discovered there was an immortality serum?
You'd either have people who would kill for it, or people who would kill to destroy it, or just that most people wouldn't believe it existed. Immortality isn't a blessing, kids; there's a reason that when we talk about those who are upon the earth forever, we describe them as being cursed. The Wandering Jew, Longinus (the centurion who pierced Christ's side with his spear), those aboard the Flying Dutchman ...
Remember that while Dunkelzahn was immensely clever, he was also rather ... hmm. Honestly, he was more than a little screwwy. Read when he visits Harlequin and talks about discussing 'what's to come' with humanity; Harlequin talks him out of it, for good reason. "A person is smart. People are dumb, panicky dangerous animals and you know it."