I agree that sometimes some people deserve to be insulted; particularly for their own good (for instance they were carelessly doing something risky and need to be reminded to take themselves off autopilot and pay attention to what they're doing.)
But I think collateral damage may be unavoidable no matter which word you use for this. Wiktionary tells me that the word 'stupid' comes from the Latin stupidus (“struck senseless, amazed”) So, stupid didn't originate as a medical diagnosis and is ostensibly a 'safe' way to question somebody's intelligence. But I don't think it actually is; when you call somebody 'stupid' you're still framing a lack of intelligence as an undesirable trait. If somebody who earnestly does lack intelligence overhears this, they might be reminded of their own limitation and feel ashamed or inadequate because of it.
Any intelligence-impugning insult can have collateral damage, whether or not that word was originally a medical diagnosis. I think the solution to this dilemma is to be aware of who might overhear you. I avoid any intelligence-impugning insult when I'm in a room with people who are actually intellectually disabled. But if nobody like that is in the room to overhear it, then I think any of the common intelligence-impugning insults are fair game if the circumstances justify it.
> But I think collateral damage may be unavoidable no matter which word you use for this ... they might be reminded of their own limitation and feel ashamed or inadequate because of it.
This issue is that when people use "retarded" as an insult, they are saying people with intellectual disabilities should feel ashamed. When they use r- as an insult, they're saying that their target is acting like they have an intellectual disability, and that that's bad.
This is distinct, though related, to criticizing people's intelligence. When you say something is stupid, you claim you're saying it's bad because it's unintelligent. Is that why you're criticizing it? Should you be criticizing things for being unintelligent? This is a more general point than the use of r- as an insult, but it is indeed a topic which is raised by avoiding r-. I don't think the broader topic should be used to avoid talking about r- though.
> I think the solution to this dilemma is to be aware of who might overhear you.
The type of impacts depend on who hears it, but you don't need a target of the insult for the insult to be bad. You are still perpetuating the idea that people with intellectual disabilities are shameful when you use r-.
