Imagine that someone offered you something and said, “This might give you a short-term performance boost. If it does, we’re not sure how long the effect will last but we know it will diminish over time. The boost might be just a year or two and it’s all but certain that it won’t last more than three or four years. Moreover, it’s extremely probable that after that you will suffer significant negative consequences FOR THE REST OF YOUR LIFE. Do you want it?”
You might take that gamble if you were a professional athlete (see, e.g., steroids), but most of us would not. In the case of smoking, illegal drugs, alcohol, and (sometimes) fatty foods, the government actively discourages us from making that choice. But when it comes to 3rd grade retention, some state governments not only are allowing the choice but requiring it.