Yes, I think it is extraordinary to extrapolate one event in 1977 to a "once every fifty years" rate
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.
Can you explain why 2010 would be a reasonable start cutoff? That doesn't make any sense to me, since it excludes most of the time that a research-origin flu pandemic was possible. We obviously haven't had a research-origin novel sarbecovirus pandemic before maybe SARS-CoV-2; but when new technological developments occur, the most similar old technologies are our best model. Nobody had ever died in a plane crash before the Wright brothers, but anyone familiar with unpowered gliders could predict the risk.