The only "problem" Debian had was that dpkg maintainer had differing opinions on how the merge should have been done and used dpkg as a vessel to express those

The only "problem" Debian had was that dpkg maintainer had differing opinions on how the merge should have been done and used dpkg as a vessel to express those opinions. Merged /usr was already available in stretch, is the default since buster, and support for non-merged /usr has been dropped in bookworm (all non-merged systems are converted on upgrade now).