We make arbitrage like this when setting pollution laws, or speed limits. The issue here is that we don't know the cost of flattening the curve. We know that GDP is strongly correlated with life expectancy, and the loss of GDP worldwide will have important sanitary impact, but it's hard to quantify this.

That being said, I'm not saying we shouldn't try to flatten the curve. Considering we don't have the data to make the right decision, we use a greedy algorithm. But I'm quite convinced that we'll be worse off in the long term. Incidentally, we choosing to save old people over poor people.