As already mentioned. It happens all the time that DM is becoming bigger topic and channel is required. For now I create channel and copy paste screenshots from DM to the channel.

Imagine that when a project or idea is brewing in the early stage, a few key people may be discussing it, but when the idea or a case is completed (sprouting and growing), it is impossible to transform some key information or ideas into an expandable one. Private/public channels allow those who join later to be connected/into the situation, ins and outs, or all history
This situation is regrettable
I personally think this is a very important function

definitely a slack feature we need

This happens to me all the time where I start a discussion about a topic in a group message and then I realize it should be in a topic private channel so I can find the information later, but MM doesn't currently have this feature.