I think the slippery-slope metaphor is now almost entirely useless, an impediment to thought rather than an aid to it. If you want to say that A makes a worse B inevitable, say so and explain why. If you want to say that acceptance of A makes it impossible in principle to reject B, say that and explain why it matters. And if you want to reject these and similar propositions, don’t rest your case on the unreliability of slippery-slope arguments in general. All this by way of saying that Jonah’s formulation is not quite right. He can’t really take the position that anyone trying to use B as an argument against A has to prove that A will inevitably lead to B. Surely if A dramatically increases the likelihood of B in some demonstrable way, and B really would be disastrous, that’s something to take into account when considering A.