IMO the difference is whether the “slope” is emotional or physical.
Exhibit A - emotional: the government has outlawed violent crime and wants to outlaw intimidation. Argument “once they outlaw intimidation, next they’ll outlaw regular insults, next they’ll outlaw criticism”. This is a bad slippery slope argument because (I’m assuming) intimidation should be outlawed. Insults and criticism should not, but are not. If the government votes in evil-gov or you encounter evil-cop, it’s as easy for them to harass you for insults and criticism, as it would be had intimidation never been outlawed.
Exhibit B - physical: the government wants to give every citizen a brain implant that can be remotely activated to stun them. This would significantly prevent crime. However, it would also be a terrible idea, because now if you get evil-gov or evil-cop, it’s significantly easier for them to remotely stun you for non-crime.
The key is that in Exhibit A, evil-gov and evil-cop face equal resistance for punishing insults and criticism regardless of whether intimidation is outlawed, because either way, people understand that intimidation should be outlawed and insults and criticism should not. More generally, moving the Overton Window to contain a “good” thing doesn’t make it contain a “bad” thing, at least not enough so that the “good” thing isn’t worth it. But in Exhibit B, evil-gov and evil-cop face ineffective resistance for stunning people for insults and criticism, because people allowed good-gov and good-cop to give them stun implants for punishing crime; whereas if evil-gov or evil-cop stepped up and said “alright, we’re going to give everyone stun implants to punish insults and criticism”, they would face effective resistance.
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Put into perspective: Visa and Mastercard using their Monopoly to effectively prevent payment for depictions of incest and rape, assuming you think that is OK, is Exhibit A. However, Visa and Mastercard having a monopoly in the first place is Exhibit B. My argument is “we should break the Visa and Mastercard monopoly (or popularize crypto) to prevent them from restricting LGBTQ and firearms etc. in the future” (this argument still applies if they’re restricting some of that now). A counter-argument is “this will allow incest depictions, hate speech, and moreover actual drug and sex trafficking*, etc.” and my counter is “those things are bad, but are they bad enough to leave us vulnerable to power shifts restricting good content in the future?” I support free speech with a similar argument**.
It’s an argument that relies on the uncertain future, but nonetheless the change here clearly and significantly decreases the probability of a bad future, because bad-gov or bad-cop must acquire power then revert the monopoly breakup; whereas the emotional example can’t even rely on the future, because if bad-gov or bad-cop acquire enough power to cause the bad thing, they would’ve just as likely acquired enough power had we not avoided causing the good thing.
* Also note these things are already exchanged with real money, and breaking up the Visa/Mastercard monopoly won’t make them legal nor stop law enforcement from tracking and prosecuting them. The more general argument is that it’s better for society to make it hard for law enforcement to prosecute crime then give them the resources to do so, but also make it hard for them to prosecute non-crime; then make it easy for law-enforcement to prosecute crime so they need less resources, but also make it easy for them to prosecute non-crime. The justification is that we spend extra resources and let some crimes avoid prosecution, in exchange for decreased risk of non-crime prosecution now and in the future.
** and that speech is mild enough, sans confidential information etc., that it shouldn't be blocked simply to content whoever says it. But even confidential information doesn't warrant e.g. a universal backdoor and filter that could be stolen and exploited by a bad actor.