I understand where you are coming from, but some messages need context to be valuable, so without context they are hard to understand. On the other hand relaying messages of other users bring up questions about authenticity of the messages (were they really sent by the original author?).
On the note of making existing chat visible to newcomers: Adb’s mailing list experiments sent you the last 3 messages of a mailing list group. Also I had quite some discussions with people from the team about this a few years ago, generally we want some feature for this use case, like when you invite new members offer to send them the last timespan X of chat history or even the whole history. We didn’t had the opportunity to make working on it a priority yet though, also because there were still questions of how much work it would be and for what solution we should go.
I think if we developed concrete ideas in Make it possible to resend chat history on adding group members we could make some progress in that area, but at the moment there are more interesting/important features for the core team, so I can’t promise anything.