I think the problem is that people try to replace one ableist word which can be used in a ton of circumstances with a drop in replacement. That will inevitably lead to more ableist language because the whole function of an insult like that is to associate the target with a Bad Type Of Person, which disparages both. Insults should be descriptive and pointed, which “stupid” never is.
The underlying thought process needs to change, not just the word.