Misconfigured servers that can amplify are only one way to DDoS attack. Even if this wasn't a problem you would still have all the other ways to DDoS, which you

Misconfigured servers that can amplify are only one way to DDoS attack. Even if this wasn't a problem you would still have all the other ways to DDoS, which you could handle easier if you could actually block the traffic. So yes, we would still need that protocol.