If the infection rate were 50% we would be close to herd immunity (which I've read would require about 70% for this virus), so that would be better news in a sense.
Unfortunately 21% is a long way from 70%, and it's taken a massive amount of death to get to that point.
The R0 value impacts herd immunity %. So if NYC is already at 50%, then it means R0 is much higher than we thought, which means herd immunity would probably be as high as +90%. With numbers like that, not shutting down would result in the entire population getting infected in the span of a month or two.
The point I was trying to make, was that for the 'open back up' crowd, they are arguing that the IFR is similar to the flu, and only vulnerable populations are really impacted. So they say we should open up and just keep vulnerable population in lockdown. But they are ignoring the implications of the R0 value in their argument. i.e. if the IFR is really as low as they think (and consequently, the infected population is as high as they think), then nothing short of a total lockdown (or very aggressive testing and contract tracing) would stop vulnerable populations from getting infected.
The herd gets substantial benefits long before you get to 70% or whatever rate for full herd immunity.
The spread starts to slow before that point. If you're walking around infected and 20% of the people you come in contact with cannot catch it from you, 20% less people are going to catch it, no matter how much you cough on them.
Unfortunately 21% is a long way from 70%, and it's taken a massive amount of death to get to that point.