I think you are right, the problem is not always a weak chick. As in my situation, where the humidity was not right and the chicks were shrink wrapped in the membrane. I have 5 that I helped out and the rest did not make it because they could not even pip or anything. I opened the eggs and saw that they were litterally stuck and could not move to pip the egg. They were not weak, it was my fault, because of the incubator. You just have to make a judgment call and go from there. I agree about breeding weak animals, I breed goats and I have had a doe born that was weaker and did not progress like she should have and I sold her to a pet only home so that she would not be bred. But I did not incubate her, she was born naturally and her sibling was perfectly fine, so I knew it was just something with her. Maybe genetic, maybe not but I did not want her passing it on to someone else. Her sibling was wethered and sold to the same person, so they live together forever (hopefully). But it is different when you incubate them and you don't do everything right and they can't get out. They can't really pass that down. It is not genetic. Hope they make it for you. I just went through this and mine are on day 5 so I know how your feeling.
Marie