What makes you think South Korea at the time of the Korean War was a "capitalistic democracy".
That is a complete strawman, it is not what I typed and not what I think.
Or than any of the sordid little dictators "the West" spent most of the Cold War propping up could be described as a "capitalistic democracy".
The character of the primary members of the western alliance e.g. USA, Canada, UK, France, West Germany, Italy was "capitalist democracies" from 1948 to 1990.
And your indeed your words imply that those sordid dictators being propped up by the west were not of the West and therefore not in themselves the key players.
I don't think the koreans (all the koreans)should be so calm. There are two dangerous circumstances going. Infighting in the imperial capital that makes a foreign war tempting. And not having that war means strategic defeat in the short term: the US eventually loses its ability to wage war in Korea without fear of some very damaging retaliation. That does not mean the US wins, but it loses its relevance (and political leverage) in South Korea. With the fears about the rise of China as the big strategic competitor, losing that position in the korean peninsula will be a big blow. Some strategists will be tempted to overthrow the game rather than let it run this course. Thus an influential "war party" probably exists that believes a war to be a sane option. And the chinese and the south koreans do seem very nervous.
Quite so.
And for the US to follow the do nothing option effective i.e. of simply letting North Korea proceed to improve its capability until its
ICBMs can nuke the continental USA is arguably dangerously negligent significantly increasing the strategic risk to the USA.
The concept that the USA can thereafter rely on deterrence assumes that the NK leadership remains objectively rational.
There is no evidence for that. People think in bubbles and the local logic within a bubble is meaningless if applied outside.
Wars have broken out on that basis. Viewed objectively it made no sense for France to start the Franco-Prussian war or
for the Argentina to invade the territory of a nuclear power, and yet those two wars happened because of internal politics.
The intelligent thing for the intelligent people to do would be for South Korea to strike a deal with China. China doesn't have a problem, really, with a united Korea, they have a problem with a country directly on their border hosting unpredictable US troops. So they both would be satisfied by an agreement that moves out the NK regime to a hospitable retirement somewhere, unification, and Korea cutting military ties with the US.
I fear that a problem is that the NK leadership can not understand the concept of a hospitable retirement.