Those that actually moved into the 'civilized' states and took them over disappeared as separate groups very quickly.
The Xianbei people, who established the Northern Wei dynasty, took more than 100 years (408-534) to "Sinicize" themselves after moved into the Han populations of Northern China, and collapsed because of a considerable amount of them
refused to be sinicized and rebelled. Looks not "quickly" at all.
One of the dynasties that succeed Northern Wei, the Eastern Wei-Northern Qi dynasty (534-577 - that's 40 more years after the more than 100 years already), was created by a
Xianbei-ized Han general. So even agrarian Chinese can adopt a steppe culture and identity.
The example that springs to mind are the Mongols, who completed the conquest of the southern Song in 1264 CE or so, proclaimed the Great Yuan Dynasty in 1271, and by 1313 Buyantu Khan had re-established the traditional Chinese imperial examinations for civil servants. That's 49 years from Mongol Conquest to Chinese administration re-established, which is pretty fast assimilation of the conquering by the conquered's techniques and practices. That's 'fairly quickly' to me.
If accepting a bureaucrat-selecting system equals to culture assimilation, then we can also safely conclude that China also assimilated Korea and Vietnam, while Byzantine assimilated the Ottomans. Sounds about right.
The Mongols had their own steppe-style governance system, Darughachi, and the re-introduction of Imperial Examinations didn't stop the Darughachi system from running. In fact, the Confucian scholar-officials and the steppe Darughachi basically worked as two parallel system in Yuan dynasty. Buyantu Khan, who restored the Imperial Examinations, also finalized the Yuan legal code, a lot of which was drawn from Mongol traditions; and he employed them to rule his Chinese subjects.
Most importantly, the adaption of Imperial Examinations also didn't stop Mongol Emperors and aristocrats continued to speak Mongolian and participate in Mongolian culture practices - in other words, didn't mark an end of their Mongolian identity. When Chinese forces drove the Mongols away in 1368, the Mongols easily returned to the steppes as well as returned to nomadic empire. Didn't sound like being assimilated at all.
Now, the other part of that whole equation is that 'assimilation' worked both ways. …But it was, of course, quite impossible to convert China into a pastoral steppe culture, so in the long run the bulk of the cultural assimilation went one way.
Throughout the Ming dynasty - the Chinese dynasty which drove away the Mongols - the Ming subjects who lived in the border regions of the empire, continued to defect to Mongols because of high taxes or harsh corvée duties.
During the reign of Jiajing Emperor (1521-1567), the Ming bureaucrats tend to persecute
the White Lotus cultists, and as a result the locals of the Shanxi Province (right next to the territories of Tümed Mongols), who were largely cultists, defected to steppes in tens of thousands. The Khan of the Tümed Mongols, Altan Khan, even crossed the Great Wall and besieged Beijing in 1550 with the help of those defected cultists. The Chinese population defected to the Tümed Mongols were so large in number, to the point which they formed a city around the Ordu of the Tümed Mongols, named Guihua (today's Hoh'hot). Nearly all these Chinese people were assimilated into Mongols after one or two generations.
There are also a great many records about how early Manchu rulers
assimilated Han Chinese into the Eight Banners, and many of these people developed a Manchu identity afterwards. It just works both ways.
The Mongol chieftains also preferred to marry Chinese noble-ladies over their own women after the fall of the Song, so the later Yuan (and Northern Yuan) Dynasties had significant Han blood - and Han mothers-in-law nannying their children.
Again, if the conquers marrying the conquereds equals to culture assimilation, then we can also safely conclude that Sabines assimilated the Romans, and Greeks assimilated the Ottomans. Sounds about right.
Also,
please check some lists about Mongolian emperors' wives before claiming like that. Most of them were actually Mongolians, or at least Koreans (Empress Gi was Korean). Only very few of them were Han Chinese, if there was any.
If we really want to talk about "bloodlines" - it is highly possible that the Ming imperial household had a Mongol blood as well.
Yongle Emperor secretly worshipped one of the consorts of his father in the Royal Ancestral Hall as his real biological mother, instead of his father's wife. According to contemporary historical accounts, this mysterious consort, called Gong Fei 碽妃, was either Mongolian or Korean. A Mongolian chronicle,
Altan Tobchi, also claimed that Yongle's mother was a Mongolian women.
In short, let me reaffirm my argument: At least in East Asia, "speedy" cultural assimilation of steppe nomads is basically a
myth.