Thinking about it a bit more, if Crossrail wasn’t going to Heathrow, I think it would probably be defensible on the basis @he_2 gives - few people would be travelling all the way through the core.
Since it is going there though, I think more people could be making long journeys than otherwise would have done and so it perhaps should have included them. Can’t remember if the Heathrow spur was an agreed thing by the time the trains were ordered or not.