I'm not really sure where the unrealistic expectation of zero dollar expense on maintenance of a bike comes from, but it seems the norm. No matter what though, it is still pennies on the dollar for any kind of expense from a car. The most expensive thing I can do for an upgrade to my bike is to have hydraulic brakes installed, and that's <$400 with paying for the labor of someone else to do it. Like you said, brake pads get replaced, but the set for my biker are <$20. Yes, it's not free and definitely non-zero, but it rounds to zero when compared to car ownership.
