Since way back in the before time - the world has been magically blessed with two Koreas.
As North Korea continues to develop both nuclear weapons and the missile technology to carry them, pressure on South Korea to take preemptive military action will gradually rise. At some point, North Korea may have so many missiles and warheads that South Korea considers that capability to be an existential threat to its security.
This is the greatest long-term risk to security and stability in Korea, arguably more destabilizing than a North Korean collapse. If North Korea does not arrest its nuclear and missile programs at a reasonably small, defensively-minded deterrent, then Southern elites will increasingly see those weapons as threats to Southern survival, not just tools of defense or gangsterish blackmail.
A missile shield would lessen the military-offensive value of North Korea’s nuclear weapons, so reducing both first-strike temptations in Pyongyang and preemptive air-strike pressure in Seoul. Unfortunately South Korea is not hardened meaningfully to ride-out Northern nuclear strikes. Missile defense in South Korea has become politicized as a U.S. plot to dominate South Korean foreign policy (yes, really) and provoke China. (Although opinion may, at last, be changing on this.) Air drills are routinely ignored.
A North Korea with dozens of nuclear missiles, possibly one hundred, some of them on submarines, would constitute a state- and society-breaking, constitutional threat to South Korea and Japan in the event of conflict. That in turn will incentivize pre-emptive airstrikes. Of course, China and the United States might be able to restrain such South Korean action. Unlike the Soviets and Americans in the Cold War, Seoul is uniquely tied to U.S. “permission” to act.
In 2010, after two North Korean actions against the South, the then-South Korean president did want to retaliate, but the Americans talked him out of it. Similarly, offensive action against the North that potentially provokes a war – as airstrikes certainly might – would unnerve China, and China’s opposition to South Korean missile defense has already altered that discussion in Seoul.
But a nuclear capability of one-hundred missiles is a whole new level of existential threat to the South (and Japan). In lieu of very robust missile defense, highly doubtful that South Korean planners would tolerate this in the long-term. Airstrikes against North Korea have been considered before (1994 and 2010 especially), and this pressure will grow again.