Clinging to "decimated" as meaning one-tenth killed is like claiming that "gay" means happy and carefree or that "nice" means subtle or trivial. In any case, the word may have originally had nothing to do with killing or punishing at all, rather meaning "to tithe." When you start to delve a little into the unstoppable evolution of word meanings, you find that word-pedants are often just making fools of themselves.

The fact that a vocal minority have given a new meaning to an existing word doesn't mean that the old meaning has to be ignored, and cannot still be used.
Also, with "word pedants often making fools of themselves" does that include the pedants who insist that only the 'modern' meaning of words is now acceptable?