You understand wrong and these common arguments can be easily seen to have incorrect arithmetic.
There are no countries with mass infections where the fatality rate is plausibly below 1%. Of course, when the health system is overwhelmed, the fatality rate can be 4-5%. But let's just look at a 1% fatality rate to start with.
The US population is 327,000,000. Seventy percent of that is 228,900,000. One percent of that 2,289,000; A little more than two million people if we assume 70% infection and an orderly health care system. But that horrible figure isn't plausible.
Because there's more. Claims you can avoid the overwhelming of the health care system are clearly false. Dividing, assuming, unrealistically that the event could be divided evenly through a year, that means that, that many people dying in a year (365) involves 6271 people dying per day and ten times that many people, ~62,000 people, arriving very sick at hospitals, per day throughout the year. And that is far, far more than could be handled by the US health care system.
Moreover, spreading the deaths evenly is inherently unrealistic, the situation of "flattening the curve" (a good thing in itself), can't stop exponential growth as such. If you decrease the doubling time of a exponential curve, you still have a situation where most of the area, most of the deaths in this case, is in the last double. So the even supposedly flattened curve won't let things go smoothly IF we assume "everyone will get this". And that promises not just 2 million but 10 millions (at the 5% death rate). If assume the US current has 10,000 infections, we approximately 12 "doublings" away from 228,900,000 infections (70% of the population). If we slow the progression to year, half the infection will still happen in the last month, with 10% of those "serious", that 763,000 needing attention per day. We're now talking corpses in the streets levels of horror. And we'd have further deaths from this breakdown situation.
And back to the lockdown scenario. My answer is that Lockdowns are absolutely necessary and thankfully it seems likely in most situations they are coming. The figures above should show that alternative is effectively going to be complete breakdown of the health care system and effectively society.