Official comment
Thank you for your feature request! We appreciate the submission, but unfortunately cannot implement every request.
At this time we will not be changing the way that broadcast emails work, as many of our customers use the current method for updates they send frequently. They can edit the email, send it to a group, edit the email again, and sent it to another group, and so on, all without having to add multiple emails to their Messages library. Because it's been such a long-standing feature that is well liked, changing it would cause too much disruption to their workflows.