How is South Korea a good example? They have a sizeable military (huge for their population), and have a high gdp.

Yes, exactly. And if the did away with that military, the whole country would become a giant gulag within a few years.

Only works if all militaries and military activity were disbanded simultaneously. War is one of those weird economic activities specifically designed to be negative-sum. No matter the outcome of a war, society on average ends up worse off.

I wonder if without war, or at least the fear of it, we'd have created the interstate highway system, or satellites, nuclear power, jet-liners, etc. No doubt we'd have probably invented something. It's not entirely clear we would've invented the same things. The threat of eminent death tends to allow governments to "rationalize" insane spending on R&D on projects that they'd otherwise not have the stomach to wait on.