You are assuming that there really is a low-risk group. As we have seen the risk of death MIGHT be lower for some people but the difference is not complete and

You are assuming that there really is a low-risk group. As we have seen the risk of death MIGHT be lower for some people but the difference is not complete and younger, healthier people are dying, too. These are people’s lives you are using to gain that herd immunity. If we had wide-spread testing, we might be able to let some people know that they have recovered from exposure and are likely safe to return to a more normal life.