Pretty simply because the change to columns from tree view is so severe (particularly for enterprise users who don't use Mac OS), taking away the Navigation section from the Product Menu will cause many more problems than solves and the new paradigm will be buggy and need user testing anyway.
Why one or the other? Give users back the tree view, while they learn the new way.
Clarification: The navigation should not be in the "Build Menu"
The "Navigation" menu called "Navigation" has been removed.
See the discourse and mutiny