Let's assume you create this ideal world of plenty.
What happens in 100 more years when the Earth's population has doubled again, and there's just not enough room/food/water - again?
OR what happens when your Gaia gets hijacked by an autocrat, and you're all out of guns?
The reason we maintain an arsenal is for when the shit hits the fan. It just so happens that coming up with a viable threat against millions of idiots (with minimal casualties) gets expensive. But it also just so happens that having such an effective arsenal brings peace to the majority or large countries in this world.
If you have no guns, then someone else will bring theirs to the party. Consider it one of the costs of maintaining society.