Sounds like an arbitrary distinction, but in any event, it was the OP who used "slippery slope" to refer to going from "congestion pricing in NYC" to "they're going to take my car."
To make it obvious: universal suffrage is a change that happened, but it wasn’t a slippery slope towards giving dogs the right to vote. Some changes result in new stases.
This is currently happening with cigarettes. Banning them at workplaces and other public places is one thing. But we live in a capitalist country that celebrates individual freedom. Or do we? Beverly Hills CA and Manhattan Beach CA have both banned the sale of cigarettes entirely. Massachusetts banned all flavored cigarettes and is trying to permanently ban the sale of cigarettes to anyone a born after a certain date.
These go beyond "normalization", it is exactly slippery slope... get a small foothold then keep expanding the position.
(Regardless, I think the answer is simple: congestion pricing is only economically viable when an area is simultaneously congested and has alternative transportation methods that would prevent the local economy from collapsing. NYC is one of a very small handful of cities in the US where this is true, although that's largely a function of 80 years of car-centric design. Maybe it will change.)