It is clear how a conclusion is necessary for an original research article as it summarized the main aspects of the procedure, experimental results, or inferences drawn from the results. But in a survey or review paper, is it necessary?
From the many survey papers I've read, not all of it include an explicit Conclusion section. Some very good surveys and reviews do end with Furture, others with Open problems. It can be noticed that the people who differ from the norm do seem to be have established reputation in their field.
Provided the paper includes a detailed Discussion section, just how necessary is it to have a Conclusion section in a survey/review paper from the viewpoint of a reviewer (not as a reader)?
A good review article makes part of its contribution by doing synthesis, even theory building, as part of the paper. So I would expect you to summarise that contribution. If you don't like calling that a Conclusion, you can call it a Summary. But you should expect a reader to learn something from any article you write; a review just relies almost exclusively on other writers for evidence, instead of just mostly like in the case of ordinary science.