I said fifty years because that roughly covers the period during which an accident similar to the 1977 flu was possible. Perhaps I should have said longer, since influenza was first cultured in 1931; but we also need some time in the freezer for the circulating virus to diverge. I don't think much changes if we say a hundred years instead.

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.