The Sinicized Xianbei had great power, but they didn't monopolize it during the Tang (the only relevant dynasty because none of the other dynasties you mention had a long period of strength even comparable to the Tang). If you look at the early leadership of the Tang, there are officials with the Xianbei surnames of Dugu, Dou, Zhangsun, Yuchi, Qutu, etc, but there are also other officials with Chinese surnames. Now, I'm sure some of these officials were like the Li and were Xianbei with Chinese surnames, but I doubt all of them were.
For example, look at just the 24 meritorious officials who were honored at Lingyan Pavilion for their contributions to founding the Tang. How many of these would you assess are Chinese and how many are assimilated Xianbei? I honestly don't know the answer to this question, and I hope you would. I can see there are two Zhangsun there, one Qutu, one Yuchi, one Li who came from a branch of the Tang ruling house ... that's five Xianbei at least, which is probably greater than their proportion in the Chinese population at the time, but not complete dominance. If you can show me that a good majority of at least these twenty-four officials were Xianbei and name them, I might be convinced that the Xianbei "monopolized" the government at this time.