[Schultzy]

One place where Koppelman (and presumably Frakt) is mistaken:

"...any slippery slope argument depends on a prediction that doing the right thing in the instant case will in fact increase the likelihood of doing the wrong thing..."

For the person arguing against doing A by invoking the slippery slope, A is NOT the "right thing" to do. The point of a slippery slope argument is to warn against making a concession. The person who is opposed to an income tax in principle would likely use the slippery slope to sway those who tend to agree with them, but are willing abandon the principle for the sake of compromise. If A was simply the "right thing" from the start, it would not likely face argument, slippery slope or otherwise.