So far, all you've proven is that the US exerts greater control over SK and Japan than China exerts over Russia. Nobody is disputing that.
Ok. One note here though. China has the capability today and even more in 5-10 years to actually utilise all that leverage. My analysis says that China in fact won't use that leverage in the short-medium term. Can't say anything for long term because that's too far for me to estimate the world geopolitics. I lean towards still not using it but I ain't putting down serious money on it either.
The problem here is your arbitrary definition of "junior partner" and your insistence on applying it to the China Russia partnership.
Junior partners have existed during the history, continue to exist today, and will exist in the future as well. If the name itself is offensive to you, which I certainly can understand why, you can just use cooperation, partner, ally or whatever else. I consider all these words as mostly PR in the modern world but anyway, I am not a hypocrite. I freely call European countries vassals and puppets, so I am consistent here when I say Russia is becoming a junior partner
Incidentally, "jumping ship" isn't the litmus test between "junior partner" and "vassal" that you seem to think it is.
Jumping ship is just one of the features. There many others.
Historically, vassals switch their allegiances all the time, and even today, SK and Japan can also switch their allegiance: it just takes more effort. In fact, if we want to take this feudalism analogy further, liege lords had to make concessions to their vassals all the time. Guess how the Magna Carta came into being?
Vassals can indeed change allegiances however there is one crucial difference. On my American Empire framework I list both Japan and S.Korea as imperial territory, not just vassals. They are a tier above vassals, they are the kind which are practically an extension of the US. Expecting them to rebel is as much realistic as expecting your arm to rebel against you and join your enemy's body. But anyway, that's my analysis, you are free to disagree with it, however I have to say that my American Empire framework is playing pretty well.
Now for other countries which are actually vassals and not imperial territory, switching allegiance is certainly possible, but is much harder nowadays as their Lord in the modern era has many ways to control them. In the past changing allegiance was relatively easy, nowadays though its a magnitude harder, and especially when your Lord is the hegemon and not a random small weak country.
However, that's all irrelevant, because you said it yourself: in order to be a junior partner, you have to have a senior partner who's driving the ship. What ship is China driving?
China is driving the ship of the national rejuvenation and the Chinese dream. Going back to my initial point, there are only interests between countries. China's number 1 interest is national rejuvenation. Thats the ship that China drives. Now if you ask, but why would Russia want to get on that ship then? Well, China as part of the big plan for becoming a superpower has laid out a series of global plans and initiatives that in totality form a Global Order with Chinese Characteristics. GDI, GSI, GCI, BRI, SCO, ADB.
Russia, contrary to what people think, has its own agency, interests, priorities, wishes and concerns. Make no mistake, if the West was smart enough it could have Russia on its own ship two decades ago. Russia boarding China's ship is part because the West pushed it in the corner, and part because it likes China's vision because it conforms with its national interests.
There is nothing weird or shocking in geopolitics. As long as you dig deep enough everything is plain to see if you know what to look for. In fact, Russia's march to China has been noticed since the early 2000s
