the euphemism treadmill is a description of linguistic patterns, not a get out of rule free card
Using ableist language as insults is always bad, even if the words seem innocuous. Some may reference the "euphemism treadmill" to try to justify their behavior, but it's crucial to understand that the treadmill is merely a linguistic observation. It does not exist to normalize ableist behavior.
There are limited examples of this effect working in reverse. Take the word "Nice" for example. Nice back in ye old medieval times used to be a synonym for "stupid" or "simple" so saying someone was "nice" was insulting. Then there was this prolonged long fad where things being very plain and straightforward was considered a good thing and "Nice/simple" gained a positive connotation. Saying someone is "simple" or a "simpleton" retains this original sort of vibe but "Nice" now just means pleasant.