I won't even argue that, it is a fact it can be changed. Bicoastal America and NW Europe managed to make a fairly large young college-ed middle class that is surprisingly docile. The issue is simply the consequences of the change and its permanence.

If you talked to any random Roman or Ancient Greek author about it, he would basically say you guys are actively trying to get decadent and expect it will work out well? To give you the most simple potential consequence: doesn't it lead to reducing courage or motivation as well? Since this is what we precisely see in the above mentioned group: a decrease of aggressivity correlates with an increase of social anxiety, timidity, shyness i..e. low courage and with the kind of attitudes where playing videogames can be primary hobby, nay, even an identity.

Of a personal experience, as my aggression levels fluctuated, so fluctuated motivation, courage, happines, self-respect and similar things with it. Not in the sense of fluctuating between aggressive and docile behavior of course, but in the sense of needing to exercise a lot of self restraint to always stay civil vs. not needing to.

You can raise the same things about its permanence. The worst outcome is a lower-aggresion group just being taken over by a higher one. Another potential impermance comes from the fluctuation of generations. My father was a rebel (beatnik), so I had only rebellion to rebel against, and my own counter-revolutionary rebellion was approved by my grandfather :)

Finally a visual type of explanation, maybe it comes accross better. You can understand human aggressivity as riding a high energy engine towards a bad, unethical direction. Having a lot of drive to do bad things. We can do two things, steer it away into a good one or just brake and turn off the engine. Everything we seem to do in this direction seems to more like braking than steering away. For example, if we were steering, we would encourage people to put a lot of drive into creative hobbies instead of hurting each other. Therefore, we would shame the living fsck out of people who don't build something. Yet we don't do this: we praise people who build, but we neglect to shame the lazy gamers. Putting it differently, we "brake" kids when they do bad stuff, but we don't kick their butts in order to do good stuff, so they end up doing nothing mostly. Every time a child or a youth would do something useful with a competitive motivation like "I'll show those lazy fscks" we immediately apply the brake. This leads to demotivation.

So in short, negative motivation can be surpressed. The issue is, it has consequences, it is probably not permanent, and really hard to replace it with a positive one. Of course I am not talking about people like us but more like the average.