Mixed feelings on this. There are a lot of reasons why the Kim regime might end in the next 50 years, but there are also a lot of reasons why it might just keep muddling along. Either way, it's an interesting thought experiment.

Around two years ago, I wrote a comment on why the incentive structure behind the relevant nations' behavior doesn't suggest that anything's going to change soon.

As a TL:DR for why no one in the region's eager to mess with the status quo:

• China doesn't want millions of desperate refugees pouring into its northeastern provinces, doesn't want millions of desperate Korean-speaking refugees pouring into its Korean-speaking northeastern provinces, and also doesn't want a unified Korea with a U.S. base on its borders.
• Russia is in the same boat. However, if you're a sneaky fuck (and Putin qualifies), you are very much aware that the U.S. presence is a great way to keep the Chinese from doing whatever they want in the region. Russia is oddly helpful in matters regarding North Korea ...
• Japan would be on the hook for a truly stupid amount of aid money if NK ever goes under. Also, in the continuing "sneaky fuck" portion of our comment, if you're worried about Chinese aspirations in the region (and there is no such thing as a Japanese politician who isn't), NK is a convenient and diplomatic excuse to push for improvements to the Japanese Self-Defense Force rather than saying what everyone is thinking out loud.
• South Korea will be in the middle of its ultimate political, economic, and social nightmare if the North Korean regime ever falls, and the fun won't stop for the better part of a century or more.
• The United States is sort of in the background to all of this and, like South Korea, its role is very complicated to describe. However, expect hundreds of billions in aid money to keep South Korea from economic collapse, hundreds of billions to rebuild NK, a security headache from worrying about where NK's missile and nuclear technology could end up, a diplomatic headache from questions over the role of a military base in a unified Korean state, etc.
• NK's elites are terrified of ending up on the gallows. Most of the people who are truly responsible for the country's present state are now beyond the reach of human justice, but I'd hate to be the person making that argument in front of an angry mob.
• No one else is going to do anything.

There are a few things I'd update or clarify from that comment two years later, but unfortunately I think most of it's still relevant.

However, if we're thinking 50 years ahead, there are a few scenarios where that incentive structure could radically change. If it does, we may see one or more influential nations become determined to end the regime's existence, or for the regime itself to engineer its own destruction:

Scenario #1: North Korea actually hits someone or something relevant with a missile. NK isn't stupid. Missile launches are typically met by aid offers from other countries within two months, so as long as the Kims have the money to build missiles, expect them to keep firing. All of their missile launches to date have been aimed at the open ocean, but this is heavily reliant on North Korean quality control, which is famous for neither quality nor control. The result of an "Oops" might not be so benign.

• Why this might not happen: In a nod to the complicated political calculus here, the country with the most incentive to ensure that North Korea never becomes a truly credible threat is NK itself. If they hit Japan, Guam, Alaska, China, Canada, or Russia, within a week's time the South Koreans will have a North Korea-shaped parking lot on their border.
• Wild card: NK hits one of the aforementioned places in a remote area with no casualties, and there's reason to believe it was a genuine accident. I can see a lot of screaming at the U.N. and fresh sanctions if this happens, but dealing with NK after deposing the Kims would be such a pain in the ass that all of the involved nations slouch back to the incentive structure from above.

Scenario #2: The U.S. vacates its bases in South Korea. A lot of China's (and to a slightly lesser extent, Russia's) motivation is its distaste for the idea of having a U.S. base in a neighboring country. Remove the base, and China will lose interest in supporting the Kim regime. That doesn't mean they'd be content to let them fall the next day (China has economic interests in NK that might not be guaranteed under a U.N. protectorate or South Korean administration), but there's a lot they'd be willing to shrug off with the prospect of no U.S. presence on the Korean peninsula.

• Why this might not happen: The American military presence in South Korea is a key part of the U.S. "pivot" to Asia. The South Korean government would likely be resistant as well. Having a U.S. base in your country is Cadillac-class insurance coverage against invasion, and (secondarily) a really bulletproof way to manipulate China's behavior.
• Wild card: The U.S. gets a base elsewhere in Southeast Asia. The most likely nation to offer if things continue the way they are is (drumroll please) ... Vietnam.

Scenario #3: Someone or something gives the North Korean elite a way out. If NK's elites and military were offered visas and resettlement for themselves and their families elsewhere, and there was a realistic way to get them out of the country safely, there would be a mass exodus. The Kims can't run NK without them.

• Why this might not happen: It'd be difficult to find a developed country anywhere that would want to offer safe harbor to the people responsible for some of the worst human rights abuses on the planet; domestic political opposition would make mincemeat of an existing government that did this. While this might be a really effective way to destabilize the Kim regime, it's also a vivid reminder that truly effective geopolitical maneuvering is often morally questionable at best.
• Wild card: Kim Jong-un is offered exile in the West and takes it, subject to conditions in his own country. If he has genuine reason to fear a revolution or coup, he will bolt.

Scenario #4: North Korea inadvertently sells nuclear or missile technology to the wrong people. This is sort of a counterpart to Scenario #1. NK doesn't have much in the way of valuable exports, and "Room 39" (the government division responsible for getting hard currency to buy luxuries for the Kims) has resorted to counterfeiting, drug trafficking, and selling weapons to unsavory people elsewhere in order to make money. If some of their stuff finds its way to a terrorist group or unstable government, and an attack on a developed country is the result, parking in North Korea will be $9.50 for the first hour and an additional $7 for each hour after that.

• Why this might not happen: This returns us to that odd point re: NK's desire to be enough of a pest to get noticed, but not enough of a pest to be worth doing something about. They have every incentive to vet their buyers because bad things happen when you become a line item in the U.S. defense budget.
• Wild card: U.S. sanctions really squeeze the bejabbers out of NK, and they get desperate enough to sell missiles or nuclear technology to someone they might have had reservations about previously.

Scenario #5: Someone comes to office who's determined to change the status quo and can't be swayed.** Incentive structures are forced to reorient themselves around "true believers" who do whatever they want to do regardless of the benefit or punishment they'll see from it. Maybe Xi Jinping is replaced by someone who really has no interest whatsoever in supporting NK. Maybe SK or Japan's next leader is someone with a family member who was kidnapped or killed by the North Koreans. Maybe the next U.S. president just really fucking hates NK. Take your pick.

• Why this might not happen: This is very uncommon behavior at the level of a nation-state. Political leaders in democratic countries are surrounded by advisers and competing power structures that moderate "true believer" impulses, and even autocratic regimes typically follow what they see as their interests. This is one of the reasons why studying the incentive structure that guides nations' behavior is more useful over the long term than saying "Merkel will do this" or "Obama will do that."
• Wild card: Can't think of any.

Scenario #6: China annexes North Korea. From an economic perspective, North Korea is sort of a Chinese province already, and maybe the regime becomes unstable enough for the Chinese to see annexation as the best of a bad set of options.

• Why this might not happen: The Chinese really, really, really don't want the responsibility of a new province with as many problems as North Korea's got, though annexation might take the form of installing a puppet government to circumvent this. Still, it's a risky maneuver even under the best of circumstances.
• Wild card: China pre-empts a Western-led invasion of NK because controlling the country themselves is preferable to the uncertainty of what'll happen to NK under U.N. or South Korean control.

Scenario #7: Kim Jong-un and/or his successor reform the political system: This is a comment all on its own and I'm running out of space. However, I will say that political analysts are unsure about this one because the North Koreans have really painted themselves into a corner with their own propaganda. Changing the regime in any serious fashion would be tantamount to admitting that the entire power structure that governs the country is illegitimate. They also know that people are more likely to revolt after a state has relaxed control and not before, so ... yeah.