I've only been using WP for about 5 years now, but I've been in the computer field for several decades. In its' present form, Customizer is nothing more than a distraction. It should NOT be merged into core yet. It isn't ready. Customizer should be completely REMOVED from core and be made into a plugin. Let those that want to use it, use it. In the mean time, the developer can continue to refine the appearance and capabilities. Once it reaches a critical mass of users of a million-plus users, then, and only then, consider merging it into core. At that point, it should have become a truly viable product.

And what is this crap in Customizer disabling the browser enlarge/reduce function? PLEASE! Even way back in Visual Basic 3 we would resize elements based on the window size. This is a basic piece of interface design that is completely ignored in Customizer.

Last, but certainly not least, please stop designing interfaces solely for use on cell phones. A valid interface should adjust to the device. Otherwise, we would all drop back to a 14-inch monitor (or smaller) and call it day.