he could have learned to control his behaviour, but I’m not sure that’s the case for a personality entirely made up of those negative traits.
But what does it even mean to be "made up of negative traits"? Would he have been completely unable to learn? To realise that, maybe, sometimes, there is benefit in not being "evil", even for entirely selfish reasons? And who says that GoodKirk couldn't also learn to be more assertive? (psychotherapy must be easily available in our socialist future, especially for high-ranking Starfleet officers, as well as plenty time off for mental health reasons)
It's not like Kirk's positive and negative traits are inherent, he wasn't born with them, but they are a result of his life's experiences. So could one make the argument that both would just need time to adjust?