I'm not sure (in honest ignorance) that this is entirely clear or obvious. How much is really publicly known about the structure of KJI's regime? Has, for instance, he regularly purged the military and ruling councils like Saddam did, and been able to reply principly on trusted family members (isn't he a bachelor?) or are there permanent cabals that he's dependent on and may have to answer to in some circumstances? Not that a military dictatorship would be that much of an improvement (or even would fully qualify as a "regime fall") but I wouldn't be so sure that a bloodless coup couldn't happen.
Much of what we know about Kim's regime comes from defectors, some of what we know comes from other sources. First off, North Korea is not a typical military dictatorship. It's more of a cult. Kim Jong Il is quite literally believed to be a deity who wields divine power. North Koreans understand that his word is law, and his power is absolute.
Also, these people have no outside information. Most believe that the world is a wasteland that covets the wealth of North Korea. Americans are actually devils who destroyed South Korea and transformed their people into demonic slaves. Kim Jong Il, the great general, born on a sacred mountain under a host of angels, is the only thing standing between North Korea and the legions of hell.
(No, he didn't actually rip that off from the DNC.)
As far as trust goes, Kim is a severe paranoid and an adherent of the Joesef Stalin's brand of 'purge early, purge often.' In North Korea, when you are suspected of insufficient loyalty, you will either be shot, or jailed. Either way, three generations of your family will be sent to the gulags for your failure.
While he may use a number of people, he doesn't need any of them per se. From every indication we have, he is every bit as powerful as his father was. Raised from birth to be a superior being, he does not view other human life as significant. He expects total unquestioning loyalty from his Koreans, yet would kill any or all of them if it suited his needs. What's worse, he feels that this is the natural order of things, a condition he is entitled to by birth. He has no puppet masters, not the Korean elites, nor the Chinese.
This makes killing him a dicey proposition. Transferring power was hard enough from father to son, but it made sense viewed through a cultlike lens. Killing off their diety won't go over well, and a military dictatorship would likely be fractured and crippled from inception. You can't plan a coup in a nation like this; if you have any power at all, you will be tested from time to time with false offers to join a secret plot to overthrow the government. The price for answering incorrectly is high.
North Korea's sanity is held together by a shoestring named Kim Jong Il. If he is overthrown, the country will likely go from stable to civil war to anarchy in about a week. The elites know that if the regime falls, they and their families will become street lamp decorations at the hands of rampaging mobs. Kim knows this, and plays this dangerous game against potential domestic enemies that could seek to remove him. 'You can't kill me without killing yourself' is a powerful defense, one that I don't see a feasible solution to.