I have an acquaintance whose daughter is now grown. But one Saturday, the daughter, then 11, didn't want to go out. Her mother was going to be gone about an hour, they lived in a rural location, so the acquaintance told her daughter not to open to the door to anyone, and to call their neighbour if there were any issues (pre widespread cell phone times). Within that hour, a man came to the door, the girl answered, and he kidnapped her. She was blindfolded, raped, and tied to a tree in the woods. Fortunately, she was, and is, a very smart girl, and a day later, managed to loosen herself enough to escape, found the road, and walked to a farmhouse. Her blindfold had not covered her eyes completely, so she could give police many details about the car she'd been travelling in, as well as a good description of the man. He lived nearby with his mother, and had recently been released from prison for rape. The car description was the reason they were able to locate him so quickly, as it was an older model and somewhat unique. The car belonged to his mother.
I'm not suggesting this would occur, obviously, but it's an almost "worst case" scenario, with a girl who was very smart and whose mother never imagined she would open a door to a stranger when instructed otherwise.
Chances are, nothing would ever occur if an 8 year old were home alone. But the list of things that can, from rather mundane to serious, is long. I would just not risk it.