A 99% cache hit rate means the database behind it is sized for 1% of your traffic. Here is what happens the moment that assumption stops holding, and how to see it on a diagram before it happens.
Retrying is the standard answer to a transient error. When the error is saturation rather than bad luck, the retry adds load at the exact moment the system needs less of it.
Three zones does not mean you can lose one for free. It means each surviving zone has to absorb half the traffic instead of a third — and the headroom for that is a number you either bought or did not.
Both take read load off the primary database, and they are not interchangeable. One scales the number of queries you can serve; the other removes the query entirely. Picking wrong is how a design ends up with both.