I very much like the Customizer but I still prefer the backend Menu system. I don't see why as a user we can't just be able to choose whether to use the frontend or backend. There are far too many 3rd party integration with the Appearance -> Menus screen that will likely never be ported to work with the customizer or not make sense in this context any more.

If they decide to take away choice because of a "you're just a user and we know better" attitude, then that's the first version of WP I won't upgrade to until there's a plugin that adds the old Appearance -> Menus screen back / links and all.

I'm all for democratizing WordPress, removing the existing Menus screen seems more like a move toward a WP dictatorship. Don't remove choice from the equation. It's mistakes like this which will cause WP to fall from it's current lofty pedestal IMO.