How do you know that what happened to the natives was so much worse than the hundreds if not thousands of similar fates that must have befallen other cultures over the last several tens of thousands of years?
Em. First, numbers of "native ancestry" are almost all self-reported which has more to do with self-identified culture than genetic ancestry.
Second, trying to argue through the use current numbers of native ancestry is nonsensical given the US has been an outlier in terms of immigration for a very long time making it impossible to compare against. For goodness sake, around a tenth of the population has been foreign born for the better part of two centuries which has had a dramatic effect on ancestry (both genetic and self-reported).