Your estimates are off for this reason:
No one has any preexisting immunity to this virus. The influenza or the common cold-causing strains of corona virus circulate in a population that is mostly immune. In a naive population, there could be hundreds of millions of cases.
I do not know off-hand what the R0 rate was for the 1917-1919 influenza pandemic, but I do know that they were converting any large building they could to hospitals. They also implemented containment measures. Millions of people died, including well over half a million in the US alone.
I know they are. I was purposely using very low numbers and a best case scenario in order to show why governments are worried. The actual numbers are what really frightens them.