You are not even "technically" correct and I have to say it's really annoying seeing this come up again and again in every HN thread where someone uses the word. Even setting aside that, yes, usage dictates meaning in English, the OED has included the definition against which you rail for 350 years.
You're wrong. And you're the worst kind of wrong: pedantically wrong. Please stop.
It used to mean that. A long, long time ago. Not for several centuries.
Now, don’t get me wrong, the etymology of words is intensely interesting. It always makes me happy to learn how words were originally used and how that meaning changed – but all that doesn’t help you decide what a word actually means right now.
The fact that people still argue argue about the only-several-centuries-old newfangled meaning of this word signifies to me that I probably shouldn't use it in my writing. If I use it to mean "reduce by one tenth" most of my readers will be confused; if I use it to mean "destroy," a vocal minority will be annoyed.
Argue against using the word. Don’t dare argue that it has a different meaning than it actually has.
Also, I think I will increase my use of decimate. If it pisses off those annoying pedants (who are perfectly aware of what is being communicated and won’t be the slightest bit confused) I will be happy. I successfully communicated and pissed off people I don’t like. Yay!
