The problem with that is... well, destroying one of the largest metropolises in the world with over 15 million inhabitants, with knock on effects on the world economy.
And even that isn't the full extent of the problem. We aren't talking about Iraq which has had crippling sanctions, a military which had been functionally obliterated fifteen years previous and large areas of its nominal territory not even in its control. NK has ballistic missiles, nukes, chemical weapons, submarines that are apparently good enough to sink a western warship in a hot zone, truly stupendous amounts of artillery, a properly functioning and reasonably modern AA infrastructure and seventy years to stockpile and prepare. If they feel pressured enough they will use the whole lot, and that means missiles potentially nuclear and if not nuclear then chemical/ biological hitting Japan and anywhere else with US bases or sympathies, submarines sinking merchant ships etc etc as well as the colossal damage done to the South from an invasion by a million man army.
And yeah, they'd lose eventually but that won't magically repair the damage done in the meantime. Whatever the west does will be specifically tailored to avoid doing anything significant because while the North talks a good messianic apocalyptico we definitely don't and have little choice other than to let the Chinese prop them up and hope they eventually sort themselves out. Everything else is just window dressing.
One nuclear EMP should largely eliminate NK as an international threat.