Adoptee here, 1970s vintage.
Many adoptees from my era did find the closed adoption process difficult from an identity standpoint - I was also born in the US where even today there are states where adoptees are not allowed to access their original birth certificates or adoption files. This is inexcusable.
There is also the outrage that women who very much wanted to, and would have been able to, successfully and safely parent their children were not supported as single mothers, due to religious and social shame. Another unmitigated disgrace.
In this era, adoptive parents were encouraged to buy into the idea that babies were like a blank slate, downplaying the genetic connection to the birth family. Misguided and wrong. Birth mothers encouraged to put their experience behind them and pretend it never happened etc. Basically - everyone lying to themselves all around. Good times, hey? (NB: ooh but don't mention anonymous egg or sperm donation because that's diffeerreeennt? I hope we see the worldwide end to this soon. In any case Ancestry etc has blown this wide open, there is literally no place to hide now.)
Research around the benefits of contact will include feedback from individuals like me who were adopted into families who were often very similar to their birth families in terms of ethnicity, education, and economic class. On one estimate up to 4 million babies were adopted in the US and Britain between 1946 and 1972. There were...sorry if this sounds crass...options to assess which healthy (white, sorry) baby went to whom.
Would contact and more openness have been achievable in these scenarios? Maybe. I wouldn't have chosen it. I would have chosen freely available and easily accessible information from 18 and updated medical information but not the confusion of 'two families' - once you have been relinquished it's hard to see how you can pretend that there is - somehow- a future relationship to be had. Adoption is about an ending, there is no use pretending it isn't, to suggest otherwise feels like gaslighting to me.
ALSO- it would have been traumatic for me personally to feel like I had to keep both my adoptive AND birth family happy in that scenario.
The pressure and expectation on the child would be immense.
Also - there will be some women who didn't have access to contraception whose decision to relinquish was the right one for them. I have wondered if it feels like being shamed all over again when they are faced with a reunion or hear about parents maintaining ongoing contact now.
The children who are up for adoption today are in a very different place, dramatically different, and it's hard to see how findings from research with earlier adoptees is applicable. It does feel to me like the basis for the conclusions comes from looking at a population of adoptees that is not reflective of the current environment.
It may be the case that adoption as we have known it needs to be re-configured which could include some contact with birth family members that are safe, as many children adopted today may have memories of these families.
The risk of being retraumatised seems dreadfully high to me though.
I can see why, as an adoptive parent, you'd be very wary, and it would have nothing to do with being an egoist wanting to erase your child's history.