You need both types of tests, exactly for those different purposes.
Just like the rate of infected people is going up exponentially, the rate of people who were infected, recover, and are no longer transmitting is going up exponentially. Just with a time lag, along with a reduction caused by the people that don’t recover.
This new test is a good step. The point is more about the error rates for positive or negative have a large impact on what it can be used for. The reasons for using the test depend on how those error rates impact achieving the goal being looked at. It’s not as simple as one error direction is better than another, since it constrains what it can be used for.
In a month or two, an after the fact test isn’t going to be useful for identifying active infections at all.
