Dunno how it was in the West but considering this is a Japanese manga made with Japanese mindset, this was common in the Sinosphere. Well, maybe except Japan itself, but China and Vietnam (can't tell for Korea) had very concentrated dynasties for the last 2000 years. At least at the beginning of dynasties when the power was firmly in the hand of emperors, they'd purge, annd I mean "kill 3 clans" kind of purge, vassals they deemed too dangerous. Two big examples were Gaozu of Han and Taizu of Ming killing those they considered to be a threat to their reign.
You think marquis was high? Han Gaozu killed kings.
