What would happen if driving drunk became legal? Nothing? That seem extremely counterintuitive.
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).
It's the same when gunloon rednecks argue that if it wasn't guns, it would be knifes, all the way down to a paperclip.
You are making the common mistake of changing a variable in isolation, without taking into account all the things that flow from your change.
These data should not be used to make the claim that attempting to increase safety is useless. Improved vehicle safety features, for example, has been compensated for by faster driving. That means the safety features didn't help lower deaths, but they did help everyone get around faster.
It's counterintuitive - changes in perception of reality are more useful than changes in reality.
Which is precisely what is being researched and done in various areas, e.g by making the road appear narrower through various visual tricks.
And it works.
Until people learn. I don't mean by that that they necessarily know they're being tricked and consciously "undo" the trick, but they are then trained to handle a tougher constraint, and thus are able to routinely do something they could not before, out of pure training. The whole danger/constraint/learning system is actually very elastic. The net result is that not only they end up driving at the same speed as before in the modified area, they actually drive faster than before in the other non-modified areas.
Note that the different countries on that graph also have legal blood alcohol limits differing by factors of at least 4. Yet that is not reflected in the death rates.
While I do not feel safe driving at the U.S. legal limit, I do feel safe driving over the Swedish legal limit (which is 1/4 the U.S. one). While I wouldn't do it in the US, I would in Sweden.