The economic toll of shelter in place vs enforced social distancing + mask wearing while keeping most businesses open is huge though. I’m not convinced quarantine is/was necessary for most of the country, including California.

I'm not sure the economic toll is necessary. We just haven't bothered to try to mitigate it. It's like the lockdown itself; do nothing for a long time, then we'll use a nuke.

The question will be what will happen in 4-5 months. If California manages a total lockdown and by July-August is back to being normally operative then the lockdown was a good choice.

On the other extreme case there might be so few cases that there will be zero herd immunity in September and a second even worse lockdown might be needed again.

So when should have China, Italy and Iran had invoked quarantine?

what you said is probably true. And it does not contradict “the cure should not be worse than the disease” statement

It does, because the statement “the cure should not be worse than the disease” insinuates we’re overreacting.