I agree with your first assertion, but would need to see some science on the second. I don't recall ever reading about negative outcomes for only children. On the contrary only children often have many advantages. More parental attention and time. More resources, all other things being equal. More money for college. More money for medical care (braces, eye care, etc.). More money and time for hobbies like sports or music.

I don't have any stake in this (no kids and unlikely to ever have any), but I feel like I've read that only children do better on many metrics than kids with siblings, all other factors being equal.