The argument is that if drunk driving became common (because it was legal) then other drivers and pedestrians would adjust their behavior according to the increased risk.
Based on that, you would expect a spurt of additional deaths early on, as we adjusted to the new risk, and then the numbers would drop again.
That sounds pretty reasonable to me, certainly if I knew that every third driver was drunk, I would behave differently on the roads (ie, would never go near them).