Yeah, this remains a thorny issue, eh? I’m forever seeing users confused about which to use, and not being able to find stuff. They are great tools, and as @HAWK says they each serve very different purposes.
I suspect that ‘Messages’ came into existence as an easy-to-bolt-on pseudo message system. Now that we have proper messages, perhaps it is time to move on?
Moving to pragmatics, I believe that the inherent negatives of this silo’ing so clearly articulated above by @stephtara could be mitigated significantly by one of these two approaches:
Search expanded to cover Topics, Messages, and Chat by default
We currently can cover Topics and Messages by appending in:all to search (but this can’t be made the default). And we can currently search Chat, but only within the Chat silo.
To my simple mind, this then seems doable - the functionality already exists (especially for Topics/Messages).
Changing ‘Messages’ to ‘Private Topics’
Messages are essentially Private Topics already, with only a few minor differences - and a different topic list UI (two actually if you include Group Messages via /group pages).
If we went the ‘whole hog’ and stopped branding them as Messages, and made Chat (which we already call ‘DM’s’) the actual messages, then we’d really be somewhere.