You’re making assumptions about what would have happened after you had emptied your wallet/your girlfriend’s purse so let’s ignore that.
The same kind of crimes happen in countries where guns aren’t prevalent and, if anything, in lower proportions with less victims of physical violence and that person might as well just have jumped on you to not give you the time to pull out your gun vs ran away with your money as soon as you gave it to them. At close range they had the advantage with their knife.
The need here isn’t guns but social programs to help the people who might resort to those means to live.