“If there is one every hundred years (on average) in a galaxy the size of the Milky Way, that’s a million every hundred million years, easily enough to suppress evolution and keep organisms at the single-cell level.”
Nah, because the average supernova would not emit enough radiation to “sterilize” an entire galaxy. I’m not sure even the largest ones imaginable could put out that much radiation, because galaxies are very large and electromagnetic radiation disperses by the inverse square law like any other radiation, so the density of the radiation drops drastically with distance from the source.
We of course have prima facie evidence of this, because we’re sitting her alive and communicating right now. So no galaxy-wide deadly supernova events could have possibly happened in the span of human history, or in the span of history of life on earth for that matter.
Exactly. No one's claiming a single supernova sterilizes the entire galaxy. But a million of them could make a huge dent in life in the galaxy. And every hundred million years, another million of them have another go at it.