Do you ever use the word “triumph” for anything other than a Roman parade, authorized by the Roman Senate, celebrating a victory by the Roman Legions? If so, you're doing the same thing using “decimate” in place of “devastate,” which as it turns out is also Latin. It grew from “devastatus,” past participle of “devastare,” from de- + vastare “to lay waste.” So anyone who says they were “devastated” when the Spurs lost Game 6 is using the word incorrectly, I suppose.
It's a good thing English is so flexible. Let's not be like the French!
The Merriam-Webster dictionary for the word decimate is:
to select by lot and kill every tenth man of
which tells me that is the definition of decimate. It is the third acceptable use of the word that widens its definition.
The same dictionary defines triumph as you have indicated, so all of us should avoid using the word incorrectly. I'm going to try and restrict my usage to the list of synonyms they suggest. Thanks for pointing that out to me!