Studies definitely show there is a correlation between secure attachment and better sense of well being later in life, but secure attachment is note the sole delimiter of later life well being. The environment the child grew up in strongly determines how the child will develop. Such factors as family closeness, the father's role in attachment, as well as experience also strongly affect the child's development and well being later in life.
I agree with everything you wrote, but I'm not so sure about the father's role in attachment bit. What happens if there is no father? Surely there will be compensation in some way (e.g. aunts and uncles filling in where the father should have been) and it may very well be possible for the child to develop a holistic well-being.
