it's not a mistake because it's not a real rule

This isn't an example of how modern English is going to the dogs. Less has been used this way for well over a thousand years—nearly as long as there's been a written English language. But for more than 200 years almost every usage writer and English teacher has declared such use to be wrong. The received rule seems to have originated with the critic Robert Baker, who expressed it not as a law but as a matter of personal preference. Somewhere along the way—it's not clear how—his preference was generalized and elevated to an absolute, inviolable rule.