I find it difficult to imagine a future in which humans aren’t making fun of impaired cognition.

I think the context is what’s most important, if anyone actually directs such language (be it retarded, idiot, etc) towards people with genuine mental impairment, that constitutes a slur. But the word ‘retarded’ literally means ‘slow’, and is still regularly used (including by myself) in scientific and technical contexts (compared to racist or homophobic slurs, which are only ever really used in a ‘slur’ kind of way).

I wouldn’t really have a problem with calling people ‘slow’ in jest, and I don’t think many would. Imo if not ‘retarded’, it’ll be something else with the same meaning.

Nah I’m guessing because it’s a popular opinion

Nobody is saying that fish are moral agents that can empathise with other beings. That doesn’t man that they’re not moral subjects; the ability to understand that one is causing harm is not a prerequisite for the ability to suffer oneself. I think everyone knows this intuitively, but it does feel good to have our less moral habits be justified by memes that we would otherwise find to be illogical.