I agree that there's a well-established pattern of city-state replacement - I don't agree that this means that pattern has to continue. There's no reason to suggest that they can't do things differently in the future. That said, I do think you're probably right and that Venice has been replaced (and with my desire for a Venice civ, I sincerely hope it so).
In other words, you've established the pattern, but not the cause for the pattern.
They could do things differently, but that's the difference between something that is observable and a theory. We have no tested theory on how they go about what they do, at least not a complete one, but we do have what is akin to an observable law. It's true, they could go ahead and do something completely out of the ordinary, but the observed relation has not only not got any examples against it as of yet, but it has
continued with Brave New World city states so far. There is, at the present time, nothing to say that it will change, and there is no reason to think it will. There is a small chance something out of the ordinary may happen, but at the end of the day there is no reason to think it will. It's not a situation where we should think it's any more than extremely unlikely that Venice hasn't been replaced.
As an aside, the line:
In other words, you've established the pattern, but not the cause for the pattern.
Describes the difference between a law and a theory (although that can't be directly applied here, this is just an aside). I always laugh when I hear nonsense like "it's just a theory" (particularly from creationists). What a lot of people don't get is that in the hierarchy of science, a law is actually lower than a theory. A law is just the observed relation, the theory is why it happens, and is more complete and better tested. That is, the Law of Gravity comes lower than the Theory of General Relativity.
There is also some speculation on the cause of the city state pattern in the thread where I initially discussed it all, but it's untestable and incomplete at this time, so there's no reason to really dig all that up.