MW had a point that decimate can still effectively be used beyond its literal meaning, but then goes positively stupid. An historic, specific contextual use is not the same as a definition. “Ovation” comes from “exult,” therefore is applicable whenever someone is exulted. “Century” means a collection of 100, and makes sense for 100 years as well as 100 soldiers.
The interesting is that “decimate” had nothing to do with Roman soldiers, but taxation. However, Cromwell did seek to destroy the Royalists, so maybe it works anyway.
But use, “devastate.”
I will use “decimate” for two reasons: it’s correct to do so, and it irritates the Grammar Nazi’s in a big way, even though I agree that “devastate” is a better choice.