You're right, it's a start, but it would have to get somewhere like 60% for it to really hurt a university.
If Duke's numbers fall even modestly, when other comparable institutions continue to see double digit increases in applications (e.g. Northwestern was up something like 18% this year), it's relative selectivity will drop. Also, what the numbers alone don't clarify is whether the drop is across the board or (as I suspect) concentrated at the upper end of the applicant pool, since those candidates will have the most equivalent alternatives. Oh, well, Duke is unlikely ever to retreat to the obscurity in which it wallowed in the days Dick Nixon went to the law school....