Those are chat services, which are obviously a different model. Twitter operates their DMs much more like mail or email. If you were allowed to delete the DMs you sent, you could easily threaten someone, then remove the offending message... which is exactly the issue they are having with harassment on the platform in its current iteration.
Maybe, instead of thinking that language is ephemeral, we should realize that what we say has lasting consequences. Maybe then we wont be so mad when we can't take what we said back.
What we say has lasting consequences, but we're not giving a deposition when socializing. We have no mechanism to show how we've evolved over time on any currently contentious issue - a weak "that was a long time ago" doesn't tend to matter. The ability to forget what is no longer important is a trait that is crucial to our brain function, and it stands to reason that it is crucial for a society.
I find Twitter DMs to be very much like a chat. It has the UI of a chat app and messages are stored on a centralized service. What makes it obviously a different model and more like email or mail?
It is indeed arguable what it looks like, but even within chat/IM applications, there is no consensus or convention on should the message be retractable or not. People shouldn't expect one or the other.
I wasn't making an argument either way about retracting messages. I was just surprised that someone would think that the DMs are obviously like e-mail when the UI looks like FB Messenger or Hangouts and message storage and transmission is controlled by a single company. I'm still curious to know how it's like e-mail.
Maybe, instead of thinking that language is ephemeral, we should realize that what we say has lasting consequences. Maybe then we wont be so mad when we can't take what we said back.