If we look at the history of flu viruses they do not spread everywhere (due to herd immunity as the biggest factor and Vaccines) and there ends up each year with a % of the population that is never exposed to it.

IOW, some have been vaccinated or have immunity otherwise. Some are protetected by herd immunity and some are exposed but never develop symptoms or actually develop flu in their bodies. Then the last group is those that are never even exposed, don't have immunity through any particular factor but everything around it keeps them from ever being exposed.

I agree with your last sentence with one little caveat that the number of TOTAL deaths isn't going to be the same by taking measures vs taking less measures. Flattening the curve can indeed slow the deaths AND lessen the total deaths because a segment of the population, similar to flu season, will never be exposed.

I don't know what that percentage is.