How is it an oversimplification if 90% of the population decrease was because of initial contact, not overt actions by the Europeans against the natives?
More importantly, the argument that American settlers and later government policies deliberately exterminated large populations of natives does not even remotely rest on this. Are you really conflating them? Are you that ignorant of the history?
You also don't seem to understand the concept of genocide, the whole point of which is that it's extra bad to kill off a population as it gets smaller.
What a disgusting excuse for a thought process. What is it do you think that's wrong with you that gives you such a deep need to ensure that your ancestors were morally pure?