All that does with a private DM group of < 7 people is create a new DM group, and the conversation is not migrated. So you're effectively starting from scratch

All that does with a private DM group of < 7 people is create a new DM group, and the conversation is not migrated. So you're effectively starting from scratch, and the new addition has no ability to go back and catch up. I can understand the reasoning for this, but its also an area where private channels are much better at.