I know the real answer is if he did that that there'd be no movie, so save your breath if that's your answer. But was there ever an in movie explanation? Elrond knew how dangerous the ring was. He knew how it needed to be destroyed to kill Sauron forever. When Isildur refused to destroy it, why not attack him and take the ring and throw it in himself? Or even kill Isildur if it came to that? Throw Isildur in with the ring. Did he underestimate how powerful Sauron was and how he could return in the future? Did he not realize just how powerful the ring was until much later?