Agreed. Maybe not in our life time, but I'm hoping at some point, every car will be self driving. This will basically eliminate drunk driving, and as you said, effectively make that argument invalid.
Furthermore, just because two things have the same outcome, does not imply the use of or purpose of that mechanism is the same. Yep, drunk drivers and just driving in general kill far more people than guns. However, the sole purpose of a car is to transport people and goods. That's it. The fact they kill humans, accidental or otherwise does not matter. It's natural due to their interactions on the roads.
A gun's sole purpose is to take the life of something. Whether it be animals or humans, it's MAIN focus has been and always will be killing. Now of course, 99% of gun owners don't actually use guns to kill humans.. But many do to kill animals. Doesn't make them any more or less harmful as these people are still responsible gun owners. It just so happens that shooting a firearm is also enjoyable when done safely. It's a fun hobby and is something pleasurable to master. That's still also independent of the fact that it's intended purpose is different than of a car's.