I do believe that a child's attachment relationship at an early age has a strong influence on his future well being. However, I do not believe that attachment is in direct correlation with a person's well being. There are far too many other factors which can not be accounted for when only looking at attachment type and a person's future. There are various environmental and hereditary factors which can shape and affect a person's life and his future regardless of his secure relationship with his mother. Thus, though a secure attachment can help a person have a better life, it does not guarentee it.
I completely agree. There are an infinite number of factors that determine someone's "well-being" in life. Trying to limit it down to a single factor in infancy in absurd. I wholly agree that there is a tendency for people who were well-attached as an infant are more likely to have a high overall well-being later in life, but implying causation is ridiculous.
I agree that a person's experience with childhood attachments have a strong impact on the person's later personality and life. But, I also believe that it's hard to make this generalization for every person because one is shaped by personal experiences. Therefore, it depends on the individual if they maintain this positive influence in their personalities, or completely change from different life experiences.