This doesn't work when someone else outside of the group initiates a conversation.
It's a fantastic feature; meaning I am never distracted on a European Monday morning by what someone sent me on an American Friday afternoon. ;)
It still has a timeout, just a longer one, and there's some caveat about the other side sending first that I forget. The only solid solution is to discuss everything in a space instead of a regular group/1:1 chat.
Getting yourself put on legal hold was also great for email/chat retention.
Not according to the OP.
And even that has a timeout, just a much longer one.