I see plenty of benefits and use cases for editable messages (as we have now by default). However, I’m concerned by the possibility of folks revising history in non-constructive ways. Seeing the original message in list archives is useful and informative. Also, knowing that your message will be un-editable in the archives is a strong motivator to think before posting (a good thing). At least it is for me.
So, can you see the full history of edited messages? If not, would it make sense to make all messages read-only?