Not saying an attack is the solution, but diplomacy has been tried for decades and hasn't worked at all thus far.
NK falling is considered terrible by all parties. South Korea can't absorb it. China can't absorb it. It's a hot potato no one wants.
That said, all-out massive attack isn't viable. there would be no way to disguise the political and military preparations for such an attack, and China wouldn't allow it anyway. Bear in mind the the South Korean capital Seoul is within medium artillery range of the border, and there are a lot of North Korean artillery pieces aimed at it. Even without a nuclear attack capability, North Korea could do them an awful lot of damage in ways that would be difficult to prevent.
It's not worth it to the U.S. or anyone else to invade, lose 30K-100K soldiers and then have to rebuild the country, and it never has been. The nuclear arms race serves partially as a deterrent against possible invasion, and partially an attempt to get people to the negotiating table for some resources by scaring Japan and N. Korea enough that they want to deal with this.
But if they had the capability, they still don't have the desire to use it as a first strike since that would give every country around + the U.S. a reason to bomb them to the ground.
What definition of "worked" are you using? Serious question.
"Diplomacy hasn't worked" is junk to say but "attacked no-one" isn't right either.