I think it is becoming increasingly established that attachment disorders are a developmental dis-order of the brain, where the lack of adequate emotional attachment to a caregiving figure has a life-long impact on the neurological formation and structure of the brain. Attachment disorders are not a behavioral or an emotional disorder as they were once thought to be. The observable impact of an attachment "disorder" is the result of a brain that has formed differently to the norm due to inadequate support during a critical period of development.
So in a very real sense therapy can't "treat" attachment disorders, anymore than therapy can "treat" physical ailments. Therapy can inform and support, it can help people recognize patterns of behavior an adapt different responses, and some forms of therapy (EMDR and brainspotting) can create alternative neural pathways that override ones that cause distress, but talk therapy itself simply can't "fix" attachment disorders. the relationship between a therapist and client can't "fix" attachment disorders. I think therapy for people with attachment disorders went very wrong for many years because they just didn't know what they didn't know then.
I think the future of therapy for people who have experienced developmental trauma including attachment trauma is exciting. They more they know about the neurology behind it the more they can find ways to adequately support people with these experiences. We can't regrow our brains "to be normal" but we can discover ways to optimize the functioning of the brains we have.