Can you quantify what you believe is a correct prior then, and explain how you got that number?

I hope you're not going to count every natural spillover since prehistory in the denominator. The technology to culture and freeze an influenza virus didn't exist before ~1930, and the technology to genetically enhance a sarbecovirus didn't exist before ~2010. The absence of pandemics with such origin before that means nothing. No one had ever suffered a cancer induced by an X-ray tube before 1904; but that doesn't mean the risk wasn't there, and Edison's assistant still died horribly.