400,000 people dying would be 0.1% of the US population.
I'm fairly sure the economy could abosrb that, particularly given the demographic bias of mortality.
416,000, primarily young men, died in WW2 of a population of 160 million.
And what about the percentage with permanent lung damage?
Or if it mutates? Or recovery immunity does not last more than a year or so?
That's without taking into account that less than half of the population will get infected.
And if you're concerned about short duration of immunity, we shouldn't drag that thing for a year, everyone should get sick faster.
Now concentrate that death toll into 4 months. Or less.