I believe that attachment is strongly correlated with positive development--in that it can stimulate emotional well-being of a child and teach the child the importance of socializing, communicating/understanding others, leading others, etc. However, there is no set rule saying that attachment guarantees a child's well-being later on in life. In some cases, attachment can actually negatively impact a child's well-being if for instance something unfortunate were to happen to the parent (causing the parent to become fatally ill or something along these lines) the child would probably spend a good deal of his/her life in grief and possibly become socially introverted. Also, too much attachment could cause negative impacts later on in the child's life when he/she grows older and tries to be more independent. Either the child will struggle in being self-reliant or the child could even totally rebel against the parent in trying to establish his/her own ways--ultimately harming the child's well-being.
Those with secure attachment who were separated from their parents in the Strange Situation experiment showed that unlike those with anxious/resistant attachment who became exceptionally distressed, the secure attachment children tended to be only mildly distressed. Your argument would be more plausible for children with anxious/resistant attachment as opposed to secure attachment. My theory is that children with secure attachment would be more likely be able to move on in a healthy way after grieving if a parent were to leave their lives unexpectedly.
