Forum posts generally can be edited, too.
Right, but forums are a much slower, more "official" communication medium, so the edit feature makes more sense.
I see chat like in-person communication where editing something already said is impossible.
I think we're at the vi/emacs point of this discussion. Editing messages works well for me and my team, and we're glad of the feature. You don't have to like it, and I wouldn't force you to edit messages. The world supports both and we both win. :)
I do wonder though, should IRC evolve to be like Slack or should it evolve idependently? I feel that the new features should be more vital, rather than "neat"... like maybe more focus on privacy and/or anonymity. Undoubtedly, I am just resisting change...