[vacri]

So... a program where near to 100% of input kids are neglected or abused that ends up with a significantly lower proportion of those kids neglected or abused is a "very weak" argument that anything has become "any better"?

If you had a drug that could save 50% of terminally ill patients, would you say "we have a very weak case that this drug saves lives"?

In order for your statement to make sense, the failure rate of foster care would have to approach 100%. Again, talk to an actual social worker about how they see foster care and whether or not they choose to use it. It's an opinion that will most likely come with caveats, but foster care is also a service they use frequently.

[Semiapies]

"near to 100% of input kids are neglected or abused"

Untrue. Children end up in the foster care system for a variety of reasons, including simple loss of parents or a woman (or teenage girl) knowing she can't care for a child.

"significantly lower proportion of those kids neglected or abused"

The evidence is not as clear on this as you'd like it to be.

To correct your example, if we had a drug that caused strokes or death in a significant percentage of cases, simply saying, "Well, many of the patients we'd give it to would otherwise suffer strokes or death," would be a quick route to an FDA rejection.