There seem to me to be three reasons for this drift in US and South Korean elite opinion over détente with North Korea.
First, the US media is obsessed with Trump. Trump has become the single biggest issue in American life. Much of the mainstream media, which he routinely damns as fake news, is happy to point out Trump’s many failures, exaggerations, lies, hyperbole, and so on. To the “resistance”, “never Trumpers”, and Western elites generally, Trump’s failure to achieve much at Singapore, coupled with his preposterous over-hyping of it, fits with a larger pattern of Trumpian flim-flam and snow-jobs, such as passing a hugely regressive tax cut while claiming to be a populist.
That Trump has emerged as genuine threat to the Western order means he must be called out relentlessly for failures such as Singapore. He gets no passes.
Second, South Koreans do not see just how destructive, divisive, and potentially authoritarian Trump is, nor is it their concern. In South Korea, American participation in the détente is necessary given the alliance. Seoul must work with what Washington gives it, and that means, today, Trump.
Where Western critics are extremely wary of giving Trump any unearned credit given the growing threat to democracy he poses, South Koreans just do not see or care that much about that. Those are issues for the Americans and West to figure out in the medium-term. South Korea has a far more pressing short-term problem – the Orwellian tyranny on its doorstep – and Trump’s participation was a necessary part of moving the outreach process along.
Third, the South Korean left is comfortable using Trump as useful idiot. This is controversial, but increasingly left-wing Korean intellectuals and outlets are cheering, however awkwardly, for Trump’s foreign policy. I have no doubt that these figures know that Trump is a corrupt, semi-authoritarian racist, but the South Korean left has never been that keen on the US presence in Korea anyway.
The Trump debate is America’s problem; once détente is wrapped up, the Americans will leave Korea anyway, so who cares about Trump? Use him to get what you need – a presidential summit with the North to jump-start détente. It was not lost on anyone that the current liberal government was happy to run military exercises against a wholly illusory Japanese threat to the Liancourt Rocks, while assenting to the cessation of Ulchi Freedom Guardian in August.
The South Korean left is very nationalist, and racist at the fringes. It is deeply divided over whether the Japan, North Korea, or the US poses the greatest threat to the peninsula.