Adding menus to the customizer is a great idea. Even if it fails, WP needs to try it out and not play it safe. In my opinion, all options that prompt visual change should be in customizer, even content editing. With development on the web moving so fast, we can't afford to have WP stuck in admin as live website editing is not even a novelty anymore. Live editing is faster and easier. If you're new to WP, learning curve is also much shorter. For us, WP "experts", these updates break our habits, but on the long term, we all profit from it, so stop complaining.