It's not the NK million man army or even its nukes that are the biggest deterrents to a US or South Korean attempted invasion/regime change, but China.
Unless China gives the nod, any attempts by the US or SK to effect regime change or unification of the Korean Peninsula by military force will be extremely likely to result in a repeat of the Korean War, with the PLA moving into NK in support of NK forces, and that is something no one wants to see.
I think people may also be over analysing the closeness of Thaek to China and that his execution is some kind of message to Beijing. If it was mainly a foreign policy disagreement, you would think that a family, even one as warped and dysfunctional as the Kim's, might be able to work out their differences without it escalating into bloodshed. Hell, even if blood was unavoidable, there would have been far subtler ways to go about it thank the melodrama NK put on.
Chubby Kim could easily have 'disappeared' dear beloved uncle and announced the old geezer passed as a result of natural causes, and given the age if the guy, most of the world and all of NK would probably have bought it. That's how you eliminate a powerful political rival if the main objective is to try to consolidate power or affect major policy changes since that does not unduly panic the purged person's supporters or the general population or give hostile foreign powers definitive indication of internal strife.
No, the way Chubs went about this, the drama of the arrest, the comical language used to denounce Thaek, and the charges spoke more of rage to me. This wasn't business, it was personal. It was not enough that Thaek died, he had to also be publically disgraced as well.
I think the most likely explanation is a fat brat who never been told 'no' by anyone other than dear leader daddy in his life absolutely hated the fact that debloved dad left dear uncle in charge when he kicked it, and that resentment has slowly built up ever since until Chubs just could not take it any longer and ended up throwing a homicidal tantrum of epic proportions.
If that is what happened, then the most worrying thing is that there is now an immature brat with potentially serious anger control issues with his finger on a nuclear launch button who either does not have enough good sense to recognise what is in his and his regime's best interests, or who cannot control his emotions well enough to do what is best for himself and his regime rather than give in to his homicidal impulses.
I think it is this possibility that worries China the most, because even enough the Americans and South Koreans do not want another Korean War with China, they will push back hard if NK provokes them enough.