It seems that claims about attachment causing better well-being cannot be proven because there may be a couple of lurking variables that can produce different results. First, children are very resilient. Many children experience trauma or difficulties, such as a divorce - yet they still manage to be well-being despite the seperation of the parents (weaker attachments). In addition we have to consider the child's environment. It is not just about the attachment style received from the parents - children who are in a supportive environment with other adults and friends caring for them can definitely lead to a better well-being, regardless of the attachment style.
You bring up a good point about the trauma and divorce that a child can undergo. I also believe that these are definitely confounding variables. I did not think of outside friends of the family forming attachments either. These extra attachments definitely help a person achieve higher well-being later in life and they can not easily be accounted for in a study. These two lurking variables are probably just a few of the multitude of issues that arise from the environment and can positively or negatively impact a person's life.
I think the fact that children are very resilient is a good point to bring up. It is easier for younger children to bounce back from major life changes/traumatic experiences, so can still adjust better later in life. I think that older children who have some major life change such as divorce might not have a better well-being later on, even if they did have secure attachment for the first year of their life simply because they are not as resilient.
