It's very easy to cast ableist accusations at imaginary scenarios.

Nobody is suggesting the word is erased from the dictionary or existing literature be modified. We're suggesting that it is more efficient communication to choose words which communicate our intended meaning instead of incorporating unintended additional anachronistic connotations. There are plenty of existing choices that are just as easily understood, we don't even need to invent new ones.

It's the same reason I no longer call cigarettes "fags" - and that actually has a different etymology to the slur, whereas "mankind" is inherently based on the gendered word "man". It's just not worth it to have to actively disambiguate the meaning, especially to someone who has some associated memories of being bullied for their sexuality.

People may not say a word with negative intentions, but when you are excluded for irrelevant historical reasons that imply you're not worth considering, it's noticeable. If you think that's not the case, walk into your next work meeting and greet them only with "Hello, ladies."

Intention and perceived intention are conveyed by the words we choose, it makes sense to be intentionally unambiguous.