As someone who did have to disrupt, the idea you out forward that this means the adoptive parents just walk away and stop parenting shows how little you know of these situations. Every adoptive parent I know who has done this has continued to play a significant role in their child's lives, maintaining the relationship as far as the child allowed it, attending meetings, advocating for their child.
I have just taken the morning off work and driven over to see DD1 where she is finishing her secondary education - it is a 30-40 minute drive each way, but when we were chatting over the weekend she showed me a swelling on her leg that I thought should be looked at. So I went to pick her up, we had breakfast together then went to a pharmacist and I bought the meds they recommended plus a few other bits and pieces for treats while she revises. Have just got home after dropping her off.
Just as a child with major health issues may end up in hospital, but that does not mean the parents have 'given up' being parents. Or have parents whose children go to boarding school given up on being parents? What if the boarding school is for special needs of some sort? Does it only 'count' if the parents have adopted?
So that comparison just doesn't stand up.
If you read the Narey report on permanence for children in care he makes the very important point that children need permanence in relationships, NOT in day to day living arrangements. Obviously they need stability day to day, but that should not prevent a child moving if another placement is better for the child, BUT if they move then it should involve planning how that child will maintain significant relationships.
Some children, after abuse in a family situation, cannot cope with living in a family situation. It is constantly triggering beyond what can be healed by therapeutic parenting and other therapeutic interventions.
As for research, well, I assume that the cases that have been studied are those at the 'mild' end of abuse/neglect, as I cannot imagine anyone claiming that a child should have to have contact with their rapist, for example, just because their rapist is a family member. Plus a good number of 'chosen to relinquish' situations. I don't think you can extrapolate from it being beneficial in a limited set of circumstances to 'it should be the norm'.
The crucial aspect that made our contact 'work' (with the disadvantages that I have mentioned above) was that the girls' first mum was 100% supportive of their relationship with me. She wanted them brought up in a stable, loving environment, and she recognised that she could not provide it. The permission that gave our daughters to love me as well, be happy without her, and get on with their lives outweighed a degree of on-going damage.