Spoiler warnings ahead. Watch the Jim Jarmusch movie if you haven’t already and you have a general dislike of spoiler.
As with all things, these comments herein are my opinions and theories. They may be wrong. I don’t claim to know for certain what the writer, actors, or producers had in mind for the story. These are the lovingly crafted theories of a fan of the film who has way too much time on his hands.
Adam and Eve are vampires who have seen a huge chunk of human history go by. Whereas Eve is a survivor who would rather nourish friendships and appreciate nature, Adam is in some kind of perpetual mourning phase that only ever finds relief in the comfort of other people. It gets so bad that at one point, he actually buys a wooden bullet made from a type of hardwood that is also used to make guitars (This becomes an important point at the end of the movie, which I have pointed out to other fans of the film who missed the connection).
On visiting her husband, Eve finds the gun and the bullet and on discovering that it was newly made and on confronting him says, “Just tell me you’re having trouble with one of the others.”
“I don’t see any others,” he replies. “Ever.”
A combination of the writing and Tom Hiddleston’s delivery leads me to the conclusion that in the universe of the film, vampires are slowly dying out. This theory is supported by other ideas within the movie.
First is the notion that humans have basically destroyed their own blood. All of the drugs and toxins of the 21st century have made their blood practically lethal to vampires, to the point where they have to go out of their way to buy purified blood. The survival oriented vampire only takes blood directly from people as an absolute last resort, the way you or I might drink water we know to be contaminated just to avoid dehydration.
Not every vampire is going to have the money or the wherewithal to set up a back ally deal with a doctor who has access to blood. And some vampires, like Ava, have no problem drinking human blood and only regret it when it backfires on them in the form of sickness and/or upsetting the people in their lives.
This supports my theory that Adam, Eve, Ava, Kit, and the lovely couple at the end of the movie, are members of yet another endangered species that humanity is slowly taking down with it.