I'm just annoyed that decimation would be a 10% layoff; standard if even weak-sauce these days. Too many people use "kill one in ten" to mean "kill them all, let God sort it out."

Be careful to check whether you're in a glass house before throwing stones - "layoff" used to mean a temporary release from employment for seasonal labour before it meant a permanent one (https://www.etymonline.com/word/layoff). "Standard" as an adjective also used to mean "being held to a standard of excellence" rather than "normal" or "average". It's ok for words to change meaning over time.

Semantic drift has always happened and will always happen in languages.

Decimation has been commonly used as a synonym for absolute destruction for a long time, being annoyed by it is wasted energy, better to let it go and accept the new meaning.

at a practical level that word hasn't meant "one in ten" for like, decades. probably just need to get used to it.

Etymology is not usage though. I get where you're coming from, but fighting vernacular is all but useless outside of academia.