I don't even think that's the point, though. I think it's more of a criticism of strict stubbornness. It's not the fact that he's immortal that's the problem, it's the fact that he/they continue to "lead" in the exact same manner, attempting to ossify a society that, on its insides, is changing. The only thing consistent is change, especially in society. Isn't the point of Foundation that, despite spectacular individuals, societies move on and change?

I feel it's the point they are trying to make with the cloned leader (I wouldn't exactly say immortal). They are saying the fall is faster because of an ossified leader, but nonetheless the fall will happen anyway, because it's not just the leader that is ossified, but the whole society.