Huh. Does that mean that the Balor in LOTR was really a Bumble? Or did Gandalf just know he would only take 20d6 damage?
Let me clarify. In the book (not the movies) Gandalf is a politician. He organizes, pushes people into doing things they wouldn't normally do. Then when things go to hell in a hand basket because Bilbo retrieved a ring that had been perfectly well hidden for centuries he bumbles around sending people hither and yon. Yes, he has a fast horse. He talks people into doing stupid things and had access to some cool resources, like a good fireworks manufacturer.
But the Balor. Gandalf does one big magic thing, breaking the bridge. Which, cool. But he can only do it by breaking his staff which as far as we know was the most magical thing about him.
But he's a fakir. His really cool stuff (fighting the balor is a good example) takes place off screen. As far as we know the balor was really a bumble and he just bounced. He also took credit for other people's achievements. For example when Merry and Pip convince the ents to help, he takes credit. Because he somehow knew they'd be captured and then escape? Right.
But on screen? When people can see what he actually does? He doesn't ever really do much magic. He uses a flash bang to scare off some goblins. In Minas Tirith he keeps mumbling about opening up that can of wupp-ass that never really materializes. He has allies like the giant eagles, but that's it.
So my whole point of this overly long post is that Gandalf is a politician and a fakir. Wait ... no that wasn't it. It's that in the fiction everybody thought Gandalf should die falling that far. Because he should, there's no logical reason why even the most powerful wizard (and not even a human one) should be able to fall that far and survive.
It would break the tenuous backdrop of a world that works like our own except with magic. It would change it from something we can relate to to something that is obviously a Bugs Bunny cartoon. That's why I wouldn't stop falling at 20d6 and the barbarian would die.