On intranet wikis with small teams talk namespaces make no sense.
So they stay unused and are confusing for the users (and intranet wiki users are not the typical internet users anyway...).
The global setting would disable the namespaces completely, just like you wouldn't define corresponding talk namespaces to custom namespaces.
- the talk namespaces would disappear on namespace lists in forms like on search
- related special pages would be disabled
- related links by the system would disappear ("my talk" in user links section, "talk"-links in "Username (Talk | contribs)"
I'm aware that this requires lots of changes on different places in the core code, but I think it's worth it.
Please note that this no dupe to https://bugzilla.wikimedia.org/show_bug.cgi?id=15085