[Huggernaut]

Not sure I totally agree that it can't be done. I have no idea whether it's at all realistic/sensible/pragmatic to create a car via agile but taking your brake pad example it would seem very alright to have a set of stories such as:

"As a driver, I can stop the car at a reasonable pace" - ok, maybe you get some awful brake pads to begin with (or maybe if you are cheeky you just say...well, air resistance does that!)

"As a driver, when the car is travelling fast, I can stop the car extremely quickly" - ok so now we probably need some reasonable brake pads.

"As a driver, I don't need to replace the stopping mechanism all the time" - ok so maybe now we need to make sure the brake pads fitted are of a certain quality.

I think you can see where this is going. Sure the car probably needs brake pads...but assuming you didn't know much about cars (something agile is suited for), at each stage you could decide you need to do something different for the stopping mechanism. Additionally, many of the qualities of the brake pads demonstrate that things are not "all or nothing" necessarily.