It's a false dichotomy. The options weren't "economic ruin" versus "mass casualties", it was "economic ruin and mass casualties" or "economic ruin without mass casualties". There simply isn't a way to keep things ticking along with a disease that kills hundreds of thousands or millions (and the US government's own estimate, which some epidemiologists view as too low, was up to 2.5m dead in a matter of a few months without aggressive intervention) and leaves a large part of the populace bedridden for weeks, particularly if that's happening in a condensed period of time. Take the areas worst affected: people are dying faster than they can be buried or cremated, and that's with intervention. How many people are having a fun night out under those circumstances? How many are continuing business as usual? No one is.