The child will not become brighter due to retention. The child will most likely not perform as well as the children in the class into which they are placed after retention. About the only reason I have found for most people to want to hold children back is that they , the teachers, hold a grudge against the child.
Children have no choice over how capable they are.
Children have no choice over the family into which they are born. Children have no choice over the problems which life leaves on their doorsteps, and which distract from their ability to concentrate on school work. In the long run, the psychological damage done to children retained is irreversible. Schools are places in which children may temporarily escape the outside world. Retaining all the poorly performing children will surely impact class sizes.