Seems like arguing semantics--what is a "mistake" if not a departure from the established convention? What is the difference between a "rule" and a "mistake"? That the "mistake" is repeated a certain amount of times and/or by some critical mass of people? How often / how many people must repeat the mistake before it becomes a rule? If it becomes a new rule, does it cease to become a mistake? The classification quickly begins to feel arbitrary.

> What is the difference between a "rule" and a "mistake"?

Your question is answered directly in Pullum's article, at length. He even gives a separate explicit example of the who-whom distinction and explains how it works and how to think about it. There is a clear reason for why this is not arbitrary.

A mistake is made from ignorance or carelessness. It is individual and won't be shared, at least not widely. A rule is applied intentionally and consistently, and it can get wide support. Importantly, making up new rules does not indicate a mental or moral deficiency like making lots of mistakes might.