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.
“Exactly. No one’s claiming a single supernova sterilizes the entire galaxy”
Then what is the point of saying one happens every hundred years? It doesn’t matter how often they happen if they only affect a limited area of local space around them. There will still be thousands and thousands of star systems completely unaffected every time there is a supernova.
Plus, the same “prima facie” evidence applies even if you try to make the argument that the aggregate effect of multiple supernovas over millions of years could sterilize a galaxy. We’re still here, so we know that didn’t happen, in the entire history of life on earth.