Well, they're probably not going to literally die from loneliness or something. They will be frightened and stressed if "Mom" isn't around a lot. That can exacerbate any health problems they have, which could lead to death. Still when Mom is not around they'll be scared, lonely and miserable. That why it's generally considered better to get more than one duck/duckling. Ducklings can also imprint on each other and keep each other company.
The above is why I have a cordial hatred for "Easter duckling" people. If parents get a child a dog or cat, the child treats it like part of the family. A duckling gets treated more like a living toy. For maybe two or three weeks or so. Then the work/mess/etc increases while the "cute factor" decreases. Meaning the duckling gets tossed out of the only home it knows to die of exposure, starvation or predation, frightened and alone.