WordPress "Default Themes" are by nature niche-oriented, limited in Customizer Options, and loved as children by their developers because of all the effort involved in their creation. Current examples:
https://github.com/WordPress/twentysixteen/issues/44
https://github.com/WordPress/twentysixteen/issues/43

Why not have the core developers create an Official WordPress Theme Builder; designed and maintained for WordPress.org users only, packed full of Customizer Options, placed on GitHub, and built on the foundation of Underscores (or any other base core structure maintained by the core developers). This new direction could either totally replace the future creation of "Default Themes", or run in Parallel with them. Here are some commercial Theme Builders to possibly learn from, during that development:

http://www.elegantthemes.com/gallery/divi/
http://headwaythemes.com/
http://www.layerswp.com/
https://thethemefoundry.com/wordpress-themes/make/
https://theme.works/
http://themifyflow.com/
https://ultimatumtheme.com/

WordPress Core Developers can call it whatever-they-want (Framework, Multi-Purpose, Theme Builder, etc.), but the idea is to offer an Official, Core Compliant, Modular, Option-Packed Theme Builder that any user can use to create her/his "Perfect Theme" as they see fit.

(This may Freak some people out, but the general idea is give the consumer what they want, not what we want to give them).