Decimated has historically meant to remove one tenth of something reflecting the Roman punishment of killing one in ten soldiers in under-performing legions. Today it is often used to describe something being significantly damaged or destroyed. Which usage is correct? Well, both.
Language is an ever-changing beast that can be hard to pin down. Dictionaries try their best to keep up but general use - and misuse - of words shapes their meaning in ways that cannot be countered by printing out clear and precise definitions.
Arguing that 'decimate' should only be used to describe the destruction of one tenth of a thing puts you in the camp of linguistic prescriptivism. An approach to language that tries to abide by rigidly defined rules.
It's the mentality that causes people to become upset when words get added to the Scrabble dictionary, forgetting that the Scrabble dictionary is little more than a rulebook for a game sold at Target.
Languages need rules to define them, to give them shape and purpose and to let them be more than the nonsense regurgitation of sounds. But there are limits to how those rules can be enforced.
Doggedly insisting on the one true way to use a language ignores all of the other uses it can and is in us. Compare how someone from Scotland speaks English to the way an American does and you can easily see that enforcement of any on set of rules doesn't work.
The same goes for changing the definition of words. Use gives definition. Words change, embrace them and see how the English language becomes richer for it.
On the other hand, this is how we ended up with 'literally' being an antonym of itself.