Unfortunately they group them oddly and I don't have time to read through the material they've been pulled from.
What I was able to find showed that 64 and under makes up roughly 83% of the population.
Per the information in Curly's link below, those 83% should have been fine to keep working and get infected and transmit it all around, with a significantly high survival rate. I do agree that there would have been deaths, I'm not sure it would have been the 2.8 million you say though. I think you are basing that on the 1% ish mortality rate, but even the WHO said that the case numbers could be completely wrong because of asymptomatic people, so we really don't have a good idea of what the mortality rate would have been.
I can see why you think we saved a lot of people though with the shutdown. I was worried about the hospitals being overtaxed should this have spiked significantly at the onset, and an overtaxed healthcare system could have led to increased amounts of death due to lack of care.
Out of curiosity, many people also had their lives ruined by the shutdown due to losing their business or lack of income. How do you calculate that into how effective the shut down was?