This will pretty much never happen, not just for pitbulls but for all the breeds originally bred for aggression. To completely eliminate aggression to background levels (for lack of a better term) would involve completely destroying the breed, which people don't allow. A lot of people are very ignorant about dogs and are only interested in their appearance. For example people like dalmatians and akitas because of the way they look, but are completely ignorant about how they were originally bred for violence, not for appearance. You could spend decades cross-breeding with various gentler breeds to create a better, non-aggressive dalmatian or akita, but people won't be interested because then they won't look like dalmatians or akitas anymore. They'll simply go to the breeders that breed ones with traditional appearance, and thus, traditional aggression.Even if the particular breeder you got your doge from specifically breeds for low aggression, it's going to take a looooong fucking time to breed the fighting ability and instinct out of a powerful breed that was specifically bred to fight.