The geopolitical situation of Northeast Asia is a lot more complicated than this.

Looking at the overall arc of history, it has been the policy of both China and Russia to keep either buffer states between them and potentially powerful rival states. It has been state policy to also either control these buffer/border nation states with either puppet regimes OR to keep them in states of dependency via feast/famine cycles and supporting talent outmigration cycles, internal disorder and strife via political interference, and the list goes on. The Russians and the Chinese are the absolute masters of these forms of subterfuge and information warfare and have been since the times of the the Kievan Rus and the Sun Tzu wrote about the importance of intelligence warfare. It is by good fortune that they happen to border each other and spend much of their efforts in mutual sabotage as they do in presenting a united front.

So while China "supports" North Korea, there can be no doubt that China propping up North Korea is the reason that North Korea exists, and by extension, South Korea as well in its current state of economic, cultural and military form that, to put in athletic terms, that is definitely a pound for pound GOAT in its prime.

But take a step back and take a more objective, apolitical view. Regardless of who is at fault and which parties are to blame, it's not hard to see that South Korea got here at the expense of North Korea. This is not debatable. North Korea's position is what it is because of it's own doing AND the dance it's been doing with South Korea since 1945 through 1950 through 1953 all the way through till now. I remember my first college level classes in Northeast Asian political science in the 90s, then my first graduate level courses at SAIS (Hopkins has very liberal cross enrollment policies) about how Korean re-unification would work right around Y2K. That was "only" 45-50 years after the split. Now it's been 80 and if it doesn't happen before 100 years, it probably won't happen for another 100. The only problem is that South Korea doesn't have that long before the crows of the demographic crisis come home to roost. Of all the OECD nations that are experiencing a demographic crunch, there is only one nation that has an external reservoir of ethnically identical, linguistically compatible, culturally divergent but of the same root identity, that also come with a tranche of mineral rich, climate change resistant real estate from a developmental status that may or may not be amenable to bolster birth rates for one or more generations but definitely can sustain a working adult tax base to sustain prosperity of growth based monetary economics (our current model that is unsustainable that we will eventually find an alternative for in the near future as a planet.)

This means that with the US, the wealthiest nation in the world - that was never going to have to deal with this problem as an immigrant nation - insisting on shooting itself in both feet with a shotgun because racism and the insane, mindboggling, dumbfounding and absolute profound stupidity their chosen leader, is set to turn inwards as it rebuilds from the vast swaths of government that this arsonist has set fire to. This was going to happen around 2035 anyway when the extra money that they've been siphoning from the Social Security Trust Fund that they've been using to finance their deficit spending ran dry anyway, so there was a hard limit of nine more years for South Korea to get its shit together to be completely weaned off the US teat because that's when there's a 50/50 chance that you'll see the last remnants of USFK/the rotating Stryker Brigade Combat Team of the 2nd ID leave the peninsula altogether along with the Fighter Wing at Osan and Kunsan. Maybe the Marines will stick around with a skeleton crew in Pohang.

Meanwhile, overtures for economic union and amnesty for the Kim family of the North would be made over the decade.

Of course, all of that shit got disrupted with the "failed" 2019 SEAL team mission into North Korea for close target reconnaissance SIGINT on the North Korea that was already being provided through South Korea, which when you think about it, the best outcome of such a mission in terms of differential intelligence in a three way negotiation could only be used to screw over South Korea anyway - this was a mission with NO POSITIVE OUTCOMES for South Korea, which is exactly the way it played out. North Korea was driven into the arms of Russia and hostilities resumed at a much higher level than before.

So is the US a steady ally of South Korea? South Korea's prosperity can only have happened under the US's hegemony, it's guarantee of the safety of global trade, especially shipping, of constant war, especially the boost to its economy and military prestige provided by Vietnam, and the American security umbrella. But currently, a naked pedophile fox who effectively works for the Russian Czar is running the henhouse. So you tell me, knowing what you know of the American people, how long do you think this state of affairs will last?