The other comment has it, but two ways to think about it. "If I have one glass of water I might have another. Then another. And so on until I drown myself."
The main one though is assuming the parties cant adjust course part way though. "If we turn this ship this way we will crash into that island in two days!"
Extropolation is different as it takes into a account changes in behaviour and real world constraints
Yes, the extrapolation is often applied to people who really want to achieve one thing going in a particular direction, so it makes sense there. "You want to do X which is not terrible evil in itself, but if we let you, you'll want to do more things like X so we don't want that precedent"
The main one though is assuming the parties cant adjust course part way though. "If we turn this ship this way we will crash into that island in two days!"
Extropolation is different as it takes into a account changes in behaviour and real world constraints