I'm probably walking into a mine field here, but I really don't understand this at all. There are plenty of more pressing issues with the UX/UI of WordPress than jamming the theme switcher and other appearance options into the customizer. If the core developers need a place to find inspiration for how a front-end customizing/editing experience could flow more effectively, there's plenty of places out there doing it right.

Look at Medium. Look at SquareSpace. Look at Maptia. Hell, even look at Wix. They all have WordPress beat hands down in this department. I think the real issue with the customizer, and lots of other front-end efforts within WordPress, is that there's really no leadership in the UX/UI department.

If you ask me, it's really a shame. The simplicity of the UI is a big reason why WordPress was attractive to me back when I first started working with it. At the time, nothing else could touch WordPress in terms of easy-of-use. The problem is that if you look at things closely, there haven't been many REAL advancements in that area over the last few years. We've taken baby steps, but with the exception of the media handling, there's really been no large advances.

I wish I had something more constructive to say about how to fix this problem, but I honestly don't. I'm not sure if the WordPress ecosystem just isn't attractive to UX/UI people or if they're not listened to when they do turn up.

Whatever the case is, I hope someone is able to stand up for logic and reason on behalf of the users soon. If all we're going to get is more of this, I think we're headed for trouble.