I read with interest your article in the Guardian. I am a children's social worker who is also an adoptive mum. We maintained a relationship with our son's foster carer and her family which continues to this day. Birthday and Christmas presents are exchanged and we have been to weddings and christenings.
We have pictures of his foster family in his bedroom - they had looked after him as a small baby up to nearly three years old, when he was placed with us. His foster carer passed away and with the foster family we supported him through that and maintained contact with her adult daughter.
Initially I admit to feeling threatened; she had been the only 'mother' figure he had known. I read a really old book by Vera Fahlberg and slept on it. My husband and I agreed his emotional wellbeing was most important - more than anything else - so we got on with it.
You are both highlighting one area, as I am sure you both know there are other parts of the adoption 'journey' that need thought as well.