There’s definitions and there’s definitions. For instance, 50 years ago the only definition for “enormity” was extremely evil. Then, because the ignorant kept using it to mean size, the dictionary folks finally included that as a secondary definition about 20 years ago. But you’d obviously argue with a fence post, so take it for what it’s worth.
Decimate has been used to mean the destruction of a great number or proportion since the mid-1600s. While it may literally be the incorrect use of the word, it has been an accepted use for more than 300 years.
I do argue with fence posts on occasion, but most of us refer to them as democrats or liberals.