Its really the frequency of future impacts we need to worry about, not past ones. These may be different for a few reasons, including depletion of the stock of NEOs, the changing character of the atmosphere, and also maybe the increasing distance between earth and moon. The last really large impact (with potential global consequences) appears to have been the Eltanin impact about 2.5 million years ago. So the efforts of NASA, ESA and others to calatlogue potential threats is in my view more important than determining the frequency of past events. Which is as you say hard, because even when you find a circular structure, you are hard pushed to pin down the cause.