The letter leaves open the possibility for a summit still happening, and the president underlined that in his public remarks. It must be difficult to give up on the prospective signature foreign policy triumph of his presidency. Yet Trump would better-served swearing off the idea of a high-stakes, mediagenic tete-a-tete leading to a fundamental breakthrough.
There’s every reason to think that the North Koreans want the on-and-off summit, whatever their bombast at the moment. If nothing else, it’s a prestige boost to Kim. And then there’s possible strategic benefits of a good meeting. Kim would presumably be deferential to Trump and tell him what he wants to hear in the hopes of a warm embrace and encouraging words at summit’s end.
If a meeting went well, South Korea would push to send humanitarian relief to the North and begin economic projects with Pyongyang. We would be hard-pressed to deny the South, and then the policy of maximum pressure would be on the way to steadily loosening pressure. If this isn’t their ultimate goal, the North Koreans have learned nothing from the last 30 years.