When you press Up Arrow, your expectations of what will happen may differ. A user coming from Slack might expect the current behavior, where you are allowed to edit your last message. A user coming from IRC, however, would like expect to cycle through his last typed messages by pressing Up Arrow one or several times. And there's an existing suggestion that it cycles through slash commands.

There is only one Up Arrow button, so the best compromise will likely be to make it user configurable. This would of course also depend on the two latter options being implemented.