To the eyes of humans living today, history is full of bad decision-making. To almost every historical event that occurred, someone can wisely say "If the leaders at the time carried out the plan this way, the results would be different, and people of today would not have to suffer through the bad consequences." Unfortunately, most of such wise comments are exclusively based on information of the said "bad consequences" available to us but not even imaginable to the decision-makers of the past.
Thus, it is not only unfair to judge events of the past through modern lenses, but any conclusions from such exercise are also completely meaningless. The political events occurring on the Korean Peninsula serve as the perfect example to illustrate the point. To the dismay of the entire world, the authoritarian regime of the North continue to flaunt a brinkmanship-based military power at the expense of trapping the necessary resources for developing the national economy and resolve an almost continuous famine.
The resulting humanitarian crisis in North Korea and instability across East Asia have caused many to regard the assistance the Chinese and Russians provided during the Korean War as the ultimate culprit. While it is true that military support from communist allies almost six decades ago is indefensible for survival of the North Korean state today, it is simply ridiculous to assume that China and Russia would have known back then what North Korea would become today. If anything, for the two regional giants, North Korea of today is as much of a diplomatic headache as it is for the allies that fought against it in that pivotal War.
What is more interesting to think about is that, in a alternative history in which Russia and China never intervened in the Korean War, a whole new set of regional problems and issues would arise today. These issues, in many cases, would be no less destructive and debilitating for the development of the region as a whole. To say in short, the disappearance of North Korea would have potentially launched a more intensive and divisive competition among great powers of the region.
To accomplish the portrayal of the alternative history, it is necessary to examine what are the consequences of the War outside its immediate military results. First, the War convinced the US of the need to build up regional counterbalance to a strong Sino-Soviet alliance. Industrial bases in Japan were quickly rebuilt as pro-American military manufacturing center of East Asia, directly resulting in the Japanese pro-WWII economic miracle. Second, US renewed military and political support of the Republic of China regime on Taiwan, culminating in the passage of the Taiwan Relations Act to guarantee its de facto independence since.
On the communist side, Chinese participation in the Korean War guaranteed economic and diplomatic sanctions from the West until the 1970s, causing catastrophic economic conditions (much like what North Korea is facing today). The conditions gave political motivation for the failed Great Leap Forward, leading to famine as well as a Mao's attempt to revive his damaged prestige in the aftermath through the Cultural Revolution. The combination of events hampered Chinese economic development for at least two decades.
Thus, the Korean War without Chinese participation would yield a completely different regional balance. Without US military needs, Japanese economic revival would not have been as strong. On the other hand, a unified Korea with a larger population and access to North Korea's mineral resources will be a much larger economy than South Korea is today. Instead of Japan's economy being five times that of Korea today, the two would be almost evenly balanced economic and military powers.
The alternative story for China will be even more dramatic. Without Chinese participation in the War, Sino-Soviet split that occurred in the late 1960s will inevitably be pushed to the early 1950s, forcing the Chinese to seek American support against the Soviets. Diplomatic normalization that happened under Nixon would have happened perhaps in the Kennedy era, in turn causing the pro-capitalist economic reforms in China to potentially happen in the late 1960s. Military invasion of Taiwan would have happened without American intervention while the Great Leap Forward and the Cultural Revolution would not have happened.
And in exchange for American non-intervention in Taiwan, China is likely to have turned a blind eye to American movements in Vietnam. In the alternative history, without Chinese support, North Vietnam would have been invaded by American troops, leading to Vietnamese unification under pro-American South Vietnam. It would not be surprising to see this Vietnam to go through similar economic development under American tutelage, jut as South Korea did in the 1970s and 1980s, and, with a population greater than unified Korea, also emerge as a major regional power.
So, with Chinese non-participation in the Korean War, today's Asia would have a unified China with an even stronger economy surrounded by three middling powers of Japan, Korea, and Vietnam, each with a 3-4 trillion dollar economy. While the common people are certainly better off in terms of standard of living, such a dense concentration of great powers in one region is bound to cause today's several bi-national territorial disputes to be even more exaggerated. In fact, it is hard to say with such balance of power, there would not have been some regional wars after the Korean War.
For instance, the Dokdo issue between Korean and Japan is largely sidelined as "diplomatic protest" by the Korean government because of its need to concentrate on North Korea. The conflict over South China Sea between Vietnam and China is rather subdued because Vietnam has little confidence to defeat a China with much greater economic and military power. And if Korea is unified, issues over Chinese-controlled Korean territories of Baekdusan and Yongbyeon are bound to be more intensely fought over compared to what the cash-strapped North Koreans can do today.
And as for America, the generally pro-American atmosphere in the entire region will force it to be neutral in such nationalistic conflicts, making the region even more prone to open conflicts for territorial control, and, worse, an expedited rise of China to the position of regional hegemony. All in all, the East Asia of the alternative history would be a much more dangerous place than it is today. The trans-regional fight for territorial control and unchecked arms race would pose much bigger threat than what Kim Jong Il can do with his brinkmanship.
The tendency for some people to simply portray a historical decision as short-sighted and completely harmful is often result of shortsightedness and lack of a more comprehensive analysis of the historical situation. As the case for North Korea illustrates, the urgency of some to nobly wish for better livelihood in North Korea cause them to ignore the greater holistic cost to the entire region if an alternative is chosen. The tendency for such people to quickly jump to conclusions on historical decision-making, therefore, can be said as useless and irresponsible.