I see you edited your post but are you telling me you thought we/they should not shut down at all? They knew the death toll would be large, just not how much, and there was a chance it could have been much higher based on early data we had.
Based on information we know now I do think we should be working very actively to reopen cautiously, and I don’t think many dispute that. I’ll ask you a how you-know-question too- How do you know the harsh measures we adopted didn’t save 500K+ lives?
There is actually no way to know that right now. Flattening the curve, strictly speaking, by definition does not save lives. It simply means that those who get infected are strung out over a longer period of time with waves or seasonal outbreaks. . Eventually deaths catch up just the same.
There are a few scenarios in which flattening the curve will potentially save lives. One is if a steep curve would have resulted in a shortage of Ventilators and other equipment and medical resources, and that is mitigated by flattening the curve, that is where lives would be saved.
However, dire predictions of medical shortages by the various models were wildly exaggerated so It is unclear if lives were saved in that sense.
Another scenario is if flattening the curve actually results in eradication of the virus rather than stringing out the casualties. That is something that is unlikely.
The third is if a vaccine is developed before the recurrent waves of virus succeed in infecting the rest of the population. In that case to flatten the curve comes out a winner, At least in terms of lives lost. It is unclear if any vaccine will be developed in enough time to make that happen.