Hm, im on day around 2500, so after the year 1100 with my own kingdom from the beginning,
I dont have this problem. Sure, the sturgians are gone, but vladian, west empire, khuzait, aserai they are changing the hole time there power.
For example.
Around day 1000 the vladians were the most powerful in my game. they had around a third of the map.
Around 500 days later, the aserai were the most powerful and vladian had around 5 thiefs left.
Again around 500 days later, khuzait were the most powerful, and aserai was beaten hard, so they were only back like beginning in south and vladians had again around 20 fiefs.
Now i would say, i am the most powerful, but south empire beat a lot of aserai and khuzait. I would say the three are even in my game, but vladians are nearly gone again, like around after 1000 days...
So i cant say its like so many here wrote. In my game the power changes the hole time...