To me it seems crazy to say having a secure attachment will mean you will have a better well-being as an adult. Secure attachments are created by good parents, and good parents also tend to raise better children. So people do not turn out better because they experienced secure attachment as a child, but rather they experienced secure attachment and have a better well-being as adults because they had good parents.
I think this is a good point. Having parents who are able to foster secure attachments means that the parents have the time and want to put in the effort to care for their children and raise them well.
I agree, being a good parent is a major component in shaping how a child will develop and grow up, and because of the good parenting during childhood, that child will tend to have a better well-being as an adult.