Strange how we all differ, which is a good thing.
I would definitely not want to keep the little one, because there I don't want his / her genetics in my flock. If I had to keep it, it would be separate from my other societies. It is normal for a hatchling to be mature enough to be able to leave the nest after about 20 days. To my thinking there must be something wrong with it's genetic makeup and I would not want the fault to be perpetuated in the offspring of my birds. If a human baby is not growing properly, doctors would run extensive tests and would 10 to 1 find something wrong with one of the major organs.
I keep birds because I love birds and I really like breeding them, but it is also important to me to breed good quality birds. I get a lot of pleasure out of watching them, and seeing their interaction, but I really get a kick out of breeding the more difficult species.
I don't know if the little one would be healthy, but I certainly not allow it to breed.