A reader scolds:

Jonah, I love to read your stuff, but stay funny, not pedantic: in the 16 centuries since the Romans used its cognate in the way you prescribe, “decimate” has also come to mean “destruction” generally. Just as a mission can be “executed” without dying; as an army can be “annhilated” without being reduced to nothing; as an army can be “beaten” without having sticks pounded on them; etc., etc., something can be decimated by 50%.