First, if we remember anything from Season 4, the cure in that tube is only enough for one person. So, if they both drink half they’ll be, what, half human?
Plus, we learned from Katherine that if you’re super old, taking the cure speeds up your aging and death. Elena has only been a vampire a few years, so she’ll be in her early twenties, but Damon would be over one-hundred. The second he takes that cure he’ll die in a few days.
Damon becoming human isn’t going to happen. The writers are just being trolls.
They’re going to have Elena take the cure, making her human.
And then they’re either going to kill her off completely, have her move on from Damon and everyone somehow, or have Damon be self-less for once in his life and, for the sake of her future happiness, have him compel her into forgetting him.