I don't think you understand what neuroplasticity means. What you are saying is that you believe in Lamarckian evolution. It doesn't work. Our brains can't evolve that way. If you want humans to evolve into a more peaceful creature you're going to have to use Nazi type selective breeding and get rid of the unwanted ones in gas chambers and mass graves. Unless women stop falling in love with sexy muscle bound brutes we're not going to breed this behaviour out.
I'm seeing Floof's argument much differently.
Humans brains are highly malleable even without changing genetically. Our brains not only go through an extraordinarily long developmental phase, they remain highly adaptable throughout our lives. According to David Eagleman, we go through two phases, one in childhood and one in adolescence, where our brains develop excessive cross-linking between neurons. In childhood this primes us to learn a language, among other things. In adolescence this makes us impulse and insecure, but as time goes by, we reinforce pathways for healthy social behaviour. We tend to blame teenagers' hormones, but that's a bit like putting the cart before the horse.
This means that, the way we choose to socialise children and teens has a huge influence on who they become as adults. As tragic as it is, some of the best evidence of this comes from the children in the child protection system, who are often extremely slow in achieving the same neurodevelopmental milestones as children who are nurtured, loved and socialised. These delays aren't just a epigenetic thing caused by malnutrition or a perpetual state or fight-or-flight, but also a lack of conversation, a lack of plain old affection, and isolation from other people. Even for normal children from harmonious households, early childhood socialisation has changed significantly over the last couple of generations: it wasn't that long ago that childcare was for the wealthy, but now it is the norm for most households, at least where I live. That's going to significantly shift the nature of our society once those children become adults.
All this is really saying is that we can make (and are making) more peaceful people without a eugenics program.