Just a total guess, because I am not an engineer or hydrologist, but from my knowledge of structures in general, seems like being a dam, that Concrete wall, built initially on top of a hill, would be attached to bedrock and that the erosion should hit that same bedrock before reaching the lake.
As I said, I could be wrong, but main logic in saying this is they were only worried this collapse causing a 30’ wall of water, if that was not solid bedrock, that 30’ wall breech would rapidly wash away all the soil between it and bedrock.
I think a good portion of the main spillway will be gone next time they shut off that flow and we get to see how big that hole has grown.
As another poster has said, I'd never consider living downriver from anything on this scale. Why put so much trust into the hands of bureaucrats that have repeatedly proven themselves to be incompetent?