Just because it's still being used in the same way in some contexts doesn't negate the fact that the word's meaning has undergone change. "gaining another meaning" is, by definition, a change.
And what happens if you extrapolate into the future? As I said, its meaning is changing. It used to mean one thing. Now it means 2. In the future, will the original term still be viewed as relevant.
50 years ago the word "Golliwog" was a fictional literrary character and a children's toy. 30 years ago it attained a second meaning as a demeaning term for a black person. Today it is only viewed as a racial pejorative.
Using the term to describe an unfortunate medical condition is patently not the same as using the term to denigrate someone.
It's not a leap. It's the result of actually thinking about it instead of just following a "monkey see, monkey do" philosophy.
You're transposing the traits of one group of people with another. While your intention may only be to pass the properties of autist onto the person you're trying to insult, you're simultanteously passing the undesirable properties of the person you're trying to insult onto the autist. The insult goes both ways whether you consciously recognise it or not.
What? If someone calls someone else "autistic" on a forum like this, it's almost certainly being used to demean. Why else call someone "autistic"? Or are you trying to argue that somehow it's a compliment?
As I said, that may not be your intent. But, by logic, you are insulting both the person you're trying to insult, as well as the autistic people.
I get what you're saying. Most people who call someone else autistic as a means of insult, don't necessarily have anything against autistic people. They're simply copying what they've seen other people do without bothering to actually think about the deeper implications of what they're saying. But think about this: How did the "insult" originate if not because whomever starting using it had a prejudice against autistic people. And by copying the behaviour, do you not recognise how you're serving to normalise the use of the term as an insult, and how that association is going to cut both ways?
It's not about degree of seriousness. It's about different meanings entirely.
At this point you've not "got" my argument because you're only seeing the way the term is used today, but you're failing to analyse consequences going forward or how the word will be viewed in 20 or 30 years time.