They have 1 missile, the military will shoot it down if they ever have too, hopefully they never have too. But we can't let a rouge country keep building ICBMs because they pose a severe threat to world peace. If we let North Korea build more and more ICBMs like if they built a missile for all 60 of their warheads or if they build more then the threat would become severe and our missile defense systems would not able to handle that many. And they have never complied with negotiations and never stopped making threats.
The only way forward for them is to just back off. But if we don't adjust our stance to the developing level of threat from this regime then they will just get more powerful and dangerous, and we will not be able to protect ourselves from them anymore if that happens.