The era of mass armies ended decades ago. Military power is now more a function of GDP than population size.
For South Korea, the only significant neighboring countries they might need to worry about are North Korea, China, and Japan. All of those have similar demographic problems.
Don't be so sure. I could easily see a country that has a pretty horrid demographic decline being invaded by a verile neighbour filled with young people who want more space. They will likely view it as taking what is rightfully theirs from their decadent and declining neighbours.
I suspect that will happen quite a bit in the next century. There is never an end to history, no matter how much we wish to believe that. It will just happen the next time around with a lot more remote killing machines and a lot of online multi-channel propaganda.
