We have old data from 1918.
This is a white swan, not a black one.
More importantly, the mortality, while much higher than flu, it's still relatively low.
Now imagine a virus as contagious as this one, but with 10% mortality over all age groups. That would be unprecedented and probably cause society meltdown.
Anecdote: someone was trying to convice me to panic about Coronavirus because "three hundred and [something] people died just today!"
$ units
8 billion / 80yr
/day
It's nearly 3 million in the US.
Again, our data for all older epidemics is applicable to the epidemic in isolation. But there is no way to accurately model how the epidemic interacts with people simply because the way people live has changed drastically from past epidemics.