The goal of North Korea is not to build a self-defense that could withstand US/NATO invasion. It's to pose a credible enough retaliation so any outside invasion

The goal of North Korea is not to build a self-defense that could withstand US/NATO invasion. It's to pose a credible enough retaliation so any outside invasion attempts would never be worth the cost. Demonstrations such as this are just North Korea declaring "we can nuke Tokyo if we want, are we really worth that?". Thereby North Korea's ruling class are insulated from being externally overthrown, and can focus all their efforts on suppressing internal threats. Unless something dramatic changes, North Korea is not going to be "wiped off the planet" anytime soon.