If the Menu Customizer plugin is approved to merge into core, Halsey outlined a plan for the removal of the old menu admin screen in favor of focusing all new development on the UI in the customizer. WordPress 4.3 would point the Menus link in the admin bar to Menus in the customizer and later releases would remove all core links to the Menus admin screen, or point them to the customizer.

That's kind of crazy pants.

The long term goal is a complete re-write of the admin interface to be mobile first, JS driven, performant, and etc. etc. okay, that makes sense - but it doesn't make sense, to me, to move everything into the customizer.

Additionally, I don't understand the benefit of seeing live updates when editing menus. "Look, Bob, Contact just moved above About! Now watch me move it back!"

That's kind of tongue in cheek, I'm not trying to be a jerk. It just doesn't seem like a great idea to me.