Here's the definition by Websters:
Definition of decimate
1 : to select by lot and kill every tenth man of decimate a regiment
2 : to exact a tax of 10 percent from
poor as a decimated Cavalier —John Dryden
3 a : to reduce drastically especially in number cholera decimated the population
Kamieniecki's return comes at a crucial time for a pitching staff that has been decimated by injuries. —Jason Diamos
b : to cause great destruction or harm to firebombs decimated the city an industry decimated by recession
Two of three definitions use it as 10%, which also happen to be the first two definitions. There are also plenty of cases where losing 10% and drastic reduction are similar values.
It's not an etymological fallacy if it's also used (and understood) in the original sense. It's only the case if the original meaning is almost never used.
"The exception proves the rule" is a pretty good example, since the original meaning is "the defect demonstrates that the ruler is functioning correctly", but my experience is that it gets used in quite different ways.
"that anyone who uses it in a modernised, popularised version must be wrong."
Often it feels like someone is trying to use a $5 word they don't quite understand to make themselves sound more sophisticated. Which usually has the opposite effect, like calling all malware "a computer virus" or insisting your computer needs defragging.
Insisting that *your* version is correct because you use it is in a particular way is just daft. I'd also strongly advise against picking a word fight with writers or journalists, since non professional users of the language tend to be be quite sloppy and imprecise.
Here's Lewis Carrol's take on all this:
"When I use a word," Humpty Dumpty said, in rather a scornful tone, "it means just what I choose it to mean- neither more nor less."
There's also nothing wrong with, when you come across a word you think means one thing, and can mean another, in finding out what is what. English was mongrel language even before being the lingua franca for the British and American empires, and the resulting word pillage :)
A few years back there was a chatshow on the wireless with a well known actor and a lady who runs a very successful logistics company. She summerised her business as "moving pallets* around" and the actor was genuinely confused, since he knew of an artist's palette** and a tasting palate*** but had never come across the wooden base. The ensuring clarifying conversation was quite entertaining.
* wooden platform for moving goods around, so a forklift can pick it up. Required for delivery any any piece of HP kit weighing more than five grams, along with two miles of plastic wrapping. Origin word means straw, then straw bed.
** board for mixing paints on or a range of colours. Origin word means spade.
*** flavors or tastes, also the roof of the mouth. Origin word means sense of taste.