You make a fair point against this. I wonder how it could be implemented as to not make the player feel like their actions don't matter.
New clans could always start at low tier, so if you manage to kill off a few high tier clans from a kingdom, you may feel a sense of progress. But then again, as
@Tryvenyal pointed out, high tier clans can be weak as well if they only have one or a few clan members left.
I guess my main point is that if we have death and dynasty, there needs to be some form of replenishment, otherwise a long game may end undramatically through clans just disappearing.