Breaking this down bit by bit, of course it may run through the whole population, but the slower it does that, the more time we have to create therapies and vaccines ultimately driving down the death rate. Getting through this faster will cause more death, period. As far as herd immunity goes, we don't actually know. There are also experts out there who don't think its possible at all.
https://www.npr.org/sections/health-shots/2020/07/24/894148860/without-a-vaccine-researchers-say-herd-immunity-may-never-be-achieved
As far as places that have seen a downturn, specifically the northeast, they have seen such because they are taking precautions.
The point of mitigation is still to flatten the curve, but the problem in America is we never really did that. We have let it sweep through our country region by region.