“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.
We’re reasoning with averages here. Ever take a statistics course?
Of course it matters how often supernovae occur. If only one occurs every ten billion years, that is not going to be enough to prevent advanced forms of life from arising somewhere in a galaxy. But if a million supernovae explode every hundred million years, that could be enough to prevent vast regions of a galaxy from ever developing multicellular life forms.
Some regions may get lucky and escape, as we may have, but most won’t. That’s the reasoning.