> But when I do try to explain why "less" and "fewer" are not interchangeable, I usually just say, "Do you want fewer syrup on your pancakes?"

This feels like a strawman: As evidenced by the fact that everyone would agree your example sentence is ungrammatical, clearly "less" and "fewer" are not strictly interchangeable, either prescriptively or descriptively.

I guess what I'm trying to say is that I don't see how your example is relevant to the cases where people do in fact use "less" to mean "fewer" (or vice versa).