I really really can't understand this Customizer obsession: everithing has to be moved in to the Customizer now, but... why?
I mean, in feew month we'll have a full REST API on which anyone could build any sort of Customizer, FrontendEditor, Realtime Preview taht could be delivered as plugin, as third party software, app, even as part of the core... why why waste so much time, energy and... words on this poor designed Cutomizer?

Or Automattic / WP core developers have user data, user requests, survey that show that the Customizer is one of the more requested features?
I wuold concentrate on Media management, editor, content curation features...

Stefano