I am woefully ignorant in these areas but as I understand it there's a power struggle over those who seek to expand the English language, and those who want to keep things pretty much where they are. M-W adheres to the "if it's used incorrectly long enough, may as well consider it valid" territory, and I believe a group like Oxford is not.

But I will promise to promote your "decimate" usage if you will help me get people to stop using "impact" as a verb (i.e. "people were impacted by the study").