are you serious????? Where did you get the info that 70% of Chinese dynasties lasted less than 50 years???
I think he is counting all the smaller states that existed during the 16 kingdoms and Northern and Southern dynasties between Jin and Sui dynasty, and also the 5 Dynasties and 10 Kingdoms that existed between Tang and Song.
Goguryo's situation is more similar to Khitans and Jurchens. Unlike the Jurchens and Mongols they are not really considered as Chinese because their destruction and eviction is more complete. (Just like how Xiongnu is considered in chinese history but they are not Chinese in a sense).
The present day Korea has more to do with Silla which is the one that eventually unified most of Korea peninsula than Goguryeo, which is actually Silla's main enemy throughout the two's history. Silla defeated the remnant of Goguryeo after the latter was significantly weakened by Tang.
The present day South Korea government is trying to create nationalistic sentiment by sticking the story that Goguryeo, which covered much greater territory than present day Koreas, as related to present day korea. They are using the fact that Goguryeo defeated Sui as justification of Korea superiority over so called Chinese, and using it as a sort of claim for their 'lost' territory.
In fact Sunflower, I hate to say this, but your South Korean government is doing just exactly what you accused the Chinese of doing. It can't get more ironical than this.
Silla should be thankful of Tang intervention to ward of Japanese invasion. Joseon should be thankful of Ming's help during the Imjin wars against again, Japanese invasion. Both are the ones that have actual ties to modern day Korean nation.
