Personally, I think it is an unhelpful response, and most likely untrue, to say to an adopted child they will never see birth parents again.
Unless the child was abandoned like those in some countries (or the UK in the distant past) where they were found at a police station or hospital and birth parents cannot be traced.
For most children in the UK their birth parents are known and many of us are in letterbox contact with one or both birth parents.
There may be some situations where the history is so traumatic that it is best to say you won't see them again. And if that applies in some cases here then I would understand that.
For us, our son's birth story was not so dramatic, we are in contact with his birth mum by letterbox and he has asked about seeing birth family. We consulted with our support and they said it was not advisable yet.
I am fairly sure at 18 or later our son will try and make contact with birth family and so for that reason we will explore this again before he hits 18 so that if he still wants to see them, and if it is possible, we will be able to be involved with that process. I would imagine after 18 there is no requirement for us to be involved but we would still offer if this helped him.
I do not know what, if any, expectations our son has of meeting his birth family, in one sense I think it is perhaps curiosity for him, and we will do our best to help him in an appropriate way. If he had been abused etc, I am sure I would feel differently; and he may or may not feel differently too of course.
The difficulty with being blunt is that it can shut down discussion coming from the child. When they are young a lot of talk comes from us, as parents - what to do/what not to do/reassurances/words to describe the world etc. As they get older we may find that discussion/thoughts/opinions coming from the child are vitally important.