You or someone you love has never been hurt/killed by someone that shouldnt have been on the road, right? Thousands of drunk idiots choose to get behind the wheel fucked up and kill people every single year. I cant imagine how many more would if we stopped punishing them until they actually killed someone. Instead of someone thinking "shit, Ive probably had like 3 too many..call me a cab" they will roll the dice. People dont fear hitting and killing someone, they fear getting pulled over and slapped with a DUI....people are inherently assholes.
That viewpoint is no different than saying we shouldnt arrest someone who picks up an automatic rifle and sprays bullets into the direction of a crowd of people, unless they manage to hit someone.