The amount of work here is amazing. I don't want to take anything away from how impressive this is.

I wish I thought the Customizer was a great idea. It's not a good place for "regular people" to work on content. It's just a different version of the bad content creation and editing process - but it still doesn't make sense to "regular people". My clients are exact those folks and I won't even teach them this. Way too confusing. They have to see the effects of CSS in their theme to understand what they are doing. If they can't see that . . . it isn't a successful process. Now we'll have two such unsuccessful processes.

This plugin is the one that should be headed for core. Instead, it doesn't seem to be a "popular kid" which confuses me. https://wordpress.org/plugins/wp-front-end-editor/

I guess I suspect why the customizer is getting all the attention and I think it is a mistake. Let me put it this way. If you want to get to that magic 50% number, you have to make content creation more intuitive than it is now and more intuitive than your competitions. This Customizer won't get you there. Those regular people are my clients and they are tired of waiting for this process to get better.

Invest resources, please, please, in that Front End Editor plugin. Busy people with other jobs can learn that!