When self-defense stops, fighting occurs. That's why you'd better know both. Aikido, at least by Ueshiba, isn't a self-defense martial art.
Honestly, why do you think Aikido is not a self defense martial art?
Do you feel that there was a time in Osensei's life when Aikido was martial and was used as self defense and Osensei decided to practice it for a different reason?