I don't think this would be good UX. Maybe when you've already typed ~20 chars or so. If it was so good at prediction from first keystroke, it'd had that info you're asking in the previous response. It could also work for short commands like "expand", "make it concise", but I can also see it being distracting for incorrect prediction.

> Typing messages in these chat apps quickly becomes tedious and autocompletions help a lot with this.

If you're on Mac, you can use dictation. focus text-input, double-tap control key and just speak.