It's dystopian...but at the same it's not. Fucking genius.
Am I the only one who thought the biting was hot? I am? I'll just go over here now...
I thought it was hot too, I just won't admit it publicly.
So maybe they do actually derive some sustenance from food and blood is an extra need over and above, vampire secret sauce kind of thing.
That's an idea I've seen before in manga, and might apply here. That vampires (or at least Shinso types) don't actually require blood to live, it's more something they crave as part of their nature.
Many people have different takes on vampirism. Some require it to live, some require it for power, some only need it once to live forever, some crave it as an indulgence, some need it to remain human...the list goes on. Here it just seems to be more love-involved craving. But I digress.