changes their mind and updates the original comment like this: “[EDIT: Actually, B makes a good argument against]”. This feature would show this information in person B’s inbox, without A having to write a separate reply to their comment.
I think separate replies are usually preferable for this. (A comment is a specific thing someone said at a particular time; you shouldn’t feel obligated to go back and edit something you already said in the past, just because some of the replies were good.)
The argument in favor of the edit is: new readers may not read through the whole thread, and you don’t want them stopping at the end of your now-outdated comment and never knowing it was rebutted and that you updated.
I think separate replies are usually preferable for this. (A comment is a specific thing someone said at a particular time; you shouldn’t feel obligated to go back and edit something you already said in the past, just because some of the replies were good.)
The argument in favor of the edit is: new readers may not read through the whole thread, and you don’t want them stopping at the end of your now-outdated comment and never knowing it was rebutted and that you updated.