They still modified their behaviour dramatically. It's similar with Japan -- that also did not have much in the way of legally compelled measures. They would

They still modified their behaviour dramatically. It's similar with Japan -- that also did not have much in the way of legally compelled measures. They would not have been constitutional there. But they still gave lots of financial supports to businesses and individuals to shut down offices and stay home. People complied voluntarily, at a large scale. (Japan was the kind of country where people chose, voluntarily, to wear face masks in flu season before COVID.) In hindsight, it seems obvious to me that voluntary supports would have been adequate in a society with strong social cohesion. No one wants to get sick; give them the means to avoid it and they'll do so. Still, there is no control population without extensive social distancing; no large group of humans anywhere in the world went screw it, business as usual, no changes. So we can't say.